Yeah, you could tow a Hobie Cat or a jet ski with almost any vehicle without many problems. But if you get into power boats you may need a more substantial tow vehicle. A 16-20ft runabout can be alot of fun on many lakes and you can get a used one for not too much $, BUT, there are always ALOT of extra costs on power boats. Nothing about them is cheap. Maintainence, parts, storage, docking fees, gas, insurance, accessories add up to a big bill in a hurry. (BOAT = Bet On Another Thousand) Also on a "plenty big" lake that may be a little undersized if it gets rough, but as long as it is not a Great Lake he shoudl be OK in most weather. If he is new to boating I would suggest a course from a local Coast Guard Auxilary (or something similiar) it is easy to get into very dangerous situations if you don't know what you are doing.
I would go with the kayak suggestion for the first year until he gets the lay of the land. Then he can step up into something that fits him best. Get something big enough to comfortably seat 4 (he will want to take friends out). Be aware the depreciation on new boats is HUGE, I would suggest used.
Stay away from Jet Skis, they are fun, but they are obnoxious. You can't take them out at night. Not good for socializing. Unless it is warm enough to swim they are not much use (shorter season).