Nobody should have just one welder. A set of torches, a decent mig and a buzzbox (AC arc) should get him going. You didn’t say what your budget was, but my first welder was a used craftsman arc welder. Rigged up an extension cord and plugged it into my mom’s stove outlet, drove her nuts, still does when I go back to see them and dad needs something fixed. This is probably your cheapest out, and I think the most fun, with the new welding rods, its really easy to lay down a good bead, but thin sheetmetal or body work is mostly out of the question, though not impossible its really easy to blow through the material. Aluminum is out too since you need a DC arc for that(more expensive and more rare). You can pick these up used really cheap, they hardly ever break. A 220volt unit is the most versatile, though I’ve talked to some people that have had reasonable success with the 110v.
If your willing to spend a bit more, a Mig welder is a lot of fun and a little more versatile than an arc welder, you don’t have to change rods and there is no slag to take off afterwards, however they are little harder to care for and break more often. You can start with flux core wire to save a few bucks and then move up to a solid wire and a tank of shielding gas later, it ends up being cheaper if your doing a lot of welding, if hes just going to be puttering around flux core would probably keep him happy. Don’t even think about buying a 110Volt unit, I’ve never had or ever even heard of someone having a good expierience with one. You’ll have a hard time finding one used, and if you do it will be really high, stay away from the chinese made stuff, its crap. Haven’t looked at them in quite a while but I would say $700-$900.
The oxy acetylene torch really isn’t that great for welding, it takes a lot of practice, but brazing is pretty easy, and you can also use it to cut up lots of metal really fast. An oxy acetylene set will go from maybe $60-$150, but then you have to add tanks, you can buy smaller ones or usually lease larger ones from a gas/welding supply company, figure around $100 a year for the tanks, but filling them is cheap.
If you do get him some form of welder, make sure that you scrounge for bits and chunks of discarded metal so that he can play on christmas morning, maybe a couple of 20ft (they will cut them so they will fit in your vehicle) pieces of square tubing from a metal supply place and some casters so that he can make a cart for his welder, heck you have a welder, why would you buy one?
Excellent gift idea, no man is complete without a welder.
Woops, UK, I’m not sure what voltages your using over there, so your mileage may vary on that one.