I'll second the free weight. If he can be trusted not to blatantly misuse the equipment he should be fine. Also, the training benefits to the free weight variety are orders of magnitude greater than a machine. Meaning - he can work like a dog on that machine and make far far better progress being a slacker on the free weight bench. In addition a bar and weights have a lot more flexability. Get a bench that can serve as a rack which will allow him to squat also. The bar can be placed on the floor for rowing or pulling from the floor. All fundemental exercises and most far better at developing a strong, muscular, athletic and balanced body than the benchpress itself.
He's young and he needn't train to failure for progress. Don't get snowed into buying a smith machine version (bar on tracks) - it's equally bad in a variety of health and functional ways.
You'll have to look some but this is a bench that allows for squats too I'm not endorsing it but it's an example (squatting is really important - maybe the most important thing he can do despite the popularity of the benchpress):
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=15281&item=7145336774&rd=1
If you really want to hook him up and keep him safe too, this is the best option. This is a basic power rack (you can find them locally I'm sure). He can take the bar in any exercise from the rests and he always has the pins to rely on should he bottom out (obviously the pins need to be below the full range of motion for the exercise but it's a nice safety bail out if need be (incidentally, great gains can be made by never going to failure, especially when young - if you need help for a program PM me). Of course with the rack you need to purchase a basic flat bench (no rack attached) to slide in when benching an example of which is the 2nd link. If I was your son and you hooked me up with this - man, I'd be thrilled:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=15281&item=7146789273&rd=1
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=15281&item=7146640845&rd=1
Bar and weight can be found locally. An olympic bar and 300lbs is generally $99 or less (don't get this stuff shipped). You can probably find a lot of this stuff locally for reasonable prices - check large sporting goods stores, Sears, used equipment stores. My only concern about outfitting a 13yo is if it's just a fad or will he stick to it. Maybe you don't want to go too all out if you think it's going to be a close rack (still, if you are going to invest in your son this isn't too bad and if he refuses to ever lift again you could sell it at a consignment shop). Anyway, those are my best ideas. If he were my son and I was reasonably sure he'd stick to it, I'd get him the powerrack setup. My son is 15 months so he's not quite there yet but when he was just crawling I'd frequently tie a rope with a brick to it to get him pulling heavy (just joking!).