( I’ve just deleted about 1 hours worth of possibly the most incoherent ramblings I would have ever posted (regarding this subject which I’ve witnessed in my own life). I should get a Nobel Prize for Editing and slapping myself in the face to bring me back to reality.)
Can you spoil and only child without trying?
Yes, but only if you are supremely insecure with yourself, your relationship with your spouse and your choices in life to 1) have one child (If this is something within your control and not infertility issues) b) to work and put the child in daycare.
Set reasonable rules and punishment for your son that you and your husband agree on. When he oversteps the boundaries, follow up with the appropriate action.(You would think this is a no brainer.)Children need and are happier within the structure of reasonable boundaries.
Do no overcompensate your having to work late with “I’ll buy him a toy.” mentality. You cannot buy their love through gifts. After awhile, they will expect the gifts all the time and become resentful when you didn’t buy the better toys and gizmo’s. ( I know you know this already you are smart, you post here, but there are newbies and dipshits out there who haven’t gotten the message. Bear with me while I’m on the soap box.)
The most important thing you can do for your child is spend time with them.( TV and movies do not count.Board and card games are big points of popularity.) The second most important thing you can do is to let go of their hand and see if they can swim on their own. Don’t put your failed hopes and dreams on the young shoulders of your child.
As for wondering if you should have a second little one. Only you can decide on that. Your son will find his outlet for siblings through classmates, neighbors, cousins, pets and if need be, (what I still use to this day) a whole slew of imaginary friends.
Sure, he will miss out on charming little things like " MOOOOM , HE’S STARING AT ME AGAIN MAKE HIM STOOOOP." but this will be one less thing to discuss in therapy anyways. 
One of our neighbors have one boy, who is 11. Whether they cannot have more or didn’t want another one is none of my business and I will never ask such a rude question. His parents are professional dog trainers on the weekend and have white collar jobs during the week. This kid is incredibly articulate, thoughtful and I cannot tell you how many times I have had a normal conversation with him and haven’t had to catch myself explaining something or talking down to his level because he is that mature. ( Or maybe I’m really that immature. It’s a tough call.)He is this way because he spends so much time with adults, but he has his friends. AND, this kid is a friggin’ genius with dogs. He made my dog, Murphy the Indifferent, do every single command the first time he met her.
If people are so rude to ask you when you are going to have another child, there are a variety of responses that I have, all rude, because a rude question deserves a rude answer:
1)My husband has a low sperm count.
2)Wow, I didn’t know our reproductive organs were worthy of your scrutiny. Honey, so and so is talking about your dick with her husband.She seems to think there is something wrong with it. Why don’t you whip it out…
Anyways. You’re son will grow up just fine as a solo child.
(I’m the youngest of five and was partially spoiled, but I deserved it
)