Other, more informed individuals will throw in their two cents (and some of it will contradict mine)....but here's what I would do:
First, I'd buy used. If he does bail...you can get more of your money back. If he doesn't and you've shopped well, then he still has some decent stuff.
Second, I think separates (head+cabinet as opposed to a combo) are the way to go for someone younger (and presumably smaller). The kind of combo that can compete with two guitarists and a drummer will either weigh a ton, out price your limit, or both.
This brings us to wattage. You'll find opinions on this that run the spectrum. I'm gonna say your son will be good with 300+ watts. There are guys here that won't leave home without 1000+ for the situation you describe. Persoanlly, I run 160 watts through 2 15's for a "full volume" practice. I can get by with 1 15 if only one guitarist shows up and he's not a moron.
Now....what speaker configuration? There's a number of them, but price and weight take their toll. I'd stick with a 4x10 or 2 2x10's. Your son should get the volume and "punch" he needs to be heard.
At any rate..those are my thoughts. There are some
screaming deals to be had on this site and ebay....so recommending specific used equipment is kinda difficult.
Personally, I'm a fan of SWR heads. The older ones can be had at criminally low prices on ebay and here in the classifieds. Cabinets are another story. You can get a pair of Aguilar 12's or Bag End 15's for $600 used...but this leaves you with close to squat for a head. Based on nothing more than the countless threads and reviews on them and the few times I've heard them live...I'd go with Avatar cabinets (new):
www.avatarspeakers.com
Sorry this is lengthy and rambling. Please take the following recommendations with a grain of salt. They apply to my tastes as a player and the products in my range of knowledge. Others will have differing views and may think that what I've stated here is total crap.
My bottom line......
Head: SWR SM-400/400s/500. Expect 400/420 watts @ 8/4 ohms in the 400 and something like 450/500 watts in the 400s & 500. Recent sales on ebay for this family of heads start at $288. Expect to pay about $350 for a nice example.
Cabinet: pick something from Avatar. I'd go with the 410 neo. It weighs 65 lbs and should be all he needs. Not to mention that it's $421
after shipping. The 212 is another decent choice and it'll save a few more bucks. Your son's desired tone (if he knows what he wants...many of us don't) would be the deciding factor on the speaker configuration.
At any rate...that slots you below $800 and should get your son stupid loud with quality components. Please be sure to check the classifieds on this site daily. There are some really great deals to be had. I hope someone else chimes in with a different perspective. Lastly...get the boy hearing protection.
Mike