Our family has had a bearded dragon for a year and a half now. I love her to pieces, but I really want to add a word of caution here.
We acquired our lizard because my daughter wanted a cage pet, one that was truly hers. She’d had a hamster for two and a half years, and because it broke my heart when he died, I insisted she get a longer-lived species of pet. We did what we thought was adequate research, went to what we thought was a reputable store for reptiles, and got an adult dragon because we weren’t confident in our ability to care for a baby.
Liz (I didn’t name her) is a lovely lizard. She spends most of her day sitting on her basking rock. When you feed her, she comes over cautiously (sure, we haven’t eaten her the last 508 days or so, but she can’t be too careful) to peer at her dish. If there are wiggly things or raspberries (go figure) in it, she gets very excited and eats them quickly. Then she stares at the dish for about half an hour before going back to the basking rock or the place where she sleeps. Once or twice a week during the summer, my daughter takes her out for a bath and a spell sunning outdoors in the baby pool. Liz spent most of last winter asleep under a rock (it’s called “brumating” and most adult dragons do it - it freaked me right out at first). This is the excitement that is living with a beardie.
How old is your son? Most small reptiles are too fragile to take a lot of careless handling, and a child under the age of, say, ten may not be able to care for them. How do you feel about cultivating crickets or other insects? I often think that we should have decided to keep crickets as pets instead, since I spend almost as much time on them as I do on the lizard. Are you willing and able to buy a variety of fresh greens and other vegetables to feed one lizard (this has actually been a real boon to my family - you can’t buy these things in quantities sufficient for only one lizard, so we’ve been eating way more green veggies ourselves). How much interaction and/or entertainment does your son hope to get from a pet? The lizard, although we now love her for her own little scaly self, is not exactly a barrel of laughs, nor can she be considered exactly expressive to someone accustomed to dealing with mammals. Is there a vet qualified to deal with exotics in your area? We have to take Liz across town (and only on certain days) to see a vet who truly knows anything about her species. How much room are you willing to devote to a pet? Although the store where we bought her claimed she needed only a 20-gallon tank, we quickly discovered that was completely inadequate and her new cage, in addition to being pretty expensive, takes up 8 square feet of our daughter’s room. Do you have other pets? If there are cats in the household, they are likely to freak the lizard out completely. We’ve had to banish the cats from our daughter’s room during the day so that they won’t jump up on Liz’s cage and stress her out. How much money are you willing to spend on housing and accessories? Our lizard, like many reptiles, requires a special UV light and heat lamps in order to survive. I have to replace a flourescent bulb that costs $23 twice a year, replace 50 watt halogen lamps about that often, and pay for the power to run those things 12 - 14 hours a day. We spent about $250 on the entire cage setup, including light fixtures and timer, and that was cheaper than it might have been because we retrofitted a cabinet ourselves instead of buying one premade. Not only does the temperature of the cage have to be kept within a specific range, we must also maintain different parts of the cage at different temperatures, hot and cool, to meet her needs. The UV lamp cannot be too far away or the lizard will not thrive - they need a specific wavelength that mimics sunlight so that they can metabolize calcium, or they will die. Many other reptiles also have this requirement.
If your son is not yet in his teens, the proper care of a reptile will obviously require a fair amount of adult assistance. Someone has to go buy crickets and maintain a habitat for them until the lizard eats them. Someone has to be sure the lights are in good working order. Someone has to scoop poop from the cage whenever necessary, and the entire thing has to be cleaned and disinfected regularly - not a simple task when you consider that you must keep rocks and branches or their equivalent in the cage for the lizard to bask and climb on.
I’m really not trying to tell you not to get a reptile. We do love ours, and she’s a valued member of the pet family. However, I didn’t expect her to be nearly this much work when we got her, and it was nerve-wracking to find that she had far more complicated needs than I’d initially expected.
If you do choose to get a reptile, be sure to consider the following:
Do you want a diurnal animal? Many lizards are nocturnal, and that’s not much fun for a kid.
How much space do you have? Although many stores will try to convince you that reptiles don’t need much room, you have to plan for the animal’s growth. I would strongly recommend that you don’t get an iguana for this reason - most people don’t have a room they can dedicate to one pet.
If your son gets bored with the reptile (and, let’s face it, reptiles aren’t exactly enthralling for a child after the novelty wears off), are you willing to take over its care for the rest of its life? With proper care, many reptiles live for ten to twenty years or more, and you might find yourself shaking crickets into a cage long after the lizard’s nominal owner has left for college.
Are you prepared for the cost? Look over the information about whatever species you’re considering and compile prices for the initial setup and ongoing care (I second http://www.anapsid.org/ as a great resource).
Can you find someone to care for the animal when you go on vacation? I had to get a separate petsitter to deal with the lizard, since our cat-and-dog sitter wasn’t comfortable dealing with the insects (although now that Liz is older, she can go a week without insects if need be).
Would your son be happier with a mammal? They’re far easier for humans to understand, and they are generally more rewarding as pets. I interpret some of our lizard’s behavior as affection, but it’s kind of a reach. I have no doubts about the feelings of our cats or dog, and even the hamster was more responsive than Liz.
If you do decide to get a reptile, please do some research into the stores in your area. I strongly suggest you not buy from chain pet stores, since they typically give very little attention to the sources from which they get their animals, and their care is often shoddy. Don’t buy babies that are very small - they’re cuter than adolescent or adult animals, but they die at an alarming rate if they’re not cared for in exactly the right way (and sometimes even if they are). Especially if you get a beardie, inquire into its bloodlines and don’t give in to any impulse you may have to get a breeding pair. Inbreeding is an increasing problem with this species in the US, due in large part to indiscriminate amateur breeding. Try to find a reputable store that specializes in reptiles. Ask them about where they get their animals and how they feed them, and pay attention to the living conditions in the store. Getting a healthy animal is half the battle in ensuring that your new pet will thrive.
Last, but not least - have you considered a nice kitty or puppy? 
