He should just wait on the handgun. It sounds like he's only got 13+ months to go. Other than portability/concealability and their cultural mystique, in practical terms of power, accuracy, and range, handguns are inferior devices to long arms in all other respects.
In the meantime, he should focus on a rifle of some kind. Probably just a .22 to get the fundamentals. As others have pointed out, there are also plenty of long guns that have all the "sex appeal" of the handgun if not more. Show him a Kel-Tec Sub2000, or a Beretta CX4 Storm if he dosen't believe you. (He'd have the 21/handgun ammo problem, but if he gets an fill-in Wally-Worlder, he can honestly say "rifle" when they ask, and they'll probably sell 9mm to him, unless he looks extremely young, or dosen't have a "clean cut" look about him…)
Maybe he should look into shooting trap at Boxhorns out in Muskego. They rent shotguns for an extra dollar over the $7 for a non-member to shoot a round of trap. They'll even give you lessons for a $20 donation to the youth program.
If he likes it, he can join a trap leauge, he dosen't need to know anybody, after he gets handicapped, they'll pair him up with people of like ability. They don't even have to shoot together.
I haven't checked recently, but Daniel Boone out by Holy Hill has "vintage military shoots" a few times a year that are open to the public
He would get waxed by the old coots with match-tuned Springfileds, handloads, and, wind meters, but he could still have fun and learn a few things even with a $99 Mosin Nagant from Dunham's… If he says he's a nwebie in advance, someone at Boone should show him the ropes.
I think if he's engaged in some kind of formal shooting activity that's done on a regular basis, like trap or Highpower shoots, and not just into owning a gun because it's "cool", his parents might come around. I think that mildly pro-gun or gun-neutral parents freak out at the idea of a handgun because of the percieved ability to stuff it in a pocket when going out and "getting in trouble", regardless of your personality or level of demonstrated responsibility.
Frankly, if he's living under his parent's roof, even after he's 21 they could forbid him from keeping a firearm, since it's thier house, thier rules, no matter how old he is.
If he can swing it so he's permitted to posess and securely store long-arms under his control at home, gets a few long guns, and at least a $99 Homak cabinet from Dunham's or Wally-World, by the time he's 21, throwing a handgun or two in there shouldn't be much of a problem if he's still living at home.
I didn't tell my parents anything, and I just "exercised my rights" started buying long guns at 18, when they eventualy found out by the time I was 21, they weren't thrilled, but being conservative and gun-neutral, it wasn't the end of the world. One night of minor peevishness on their part, some negotiating, then it never came up again. Since I wanted a new car and needed their co-signature, I agreed to sell a few guns for the down payment, and I went and got a security cabinet for the rest of my guns. Since it was already water under the bridge, after that, I was able to throw whatever I wanted in there until I moved out of the house at 23.
Since his parents were willing, even temporarily, to buy him a handgun, he could probably go this route. If not, he should get started with long guns, and you can store them for him.