I don't think there's much that the above posters haven't already covered, but all I can say is beware. If the pony is sweaty or tired, it may be that he is very high strung and the owner has lunged him or ridden him previously before you came to look at him so that he would be calm. Also, bring a trainer and a vet with you. Have them give you their opinions on the pony's conformation, temperment, and decency. Also, have someone else ride or lunge the pony so that you can see how he moves.
Try leading the pony by hand and see how he behaves, and handle him from the ground. He should behave 100% perfect with you, because if he does not respect you, he will not respect your son.
I agree that you shouldn't get a pony for your son until he is at least 6. At such a young age, he does not currently have enough control of his body or muscle to properly control his mount. If you get a pony now, it will be more of your pony than his. You'll do the work, and he'll sit on it's back every now and then.
But do not completely dismiss the idea of lessons. I know for a fact that some barns - mine in particular, but I'm sure their are others - have great children's mounts that are only 11 hh smallest. They do tend to get a little stubborn and lazy at times (ponies tend to do that) but your son will probably have someone to lead him around by hand most of the time until he gains full control, and they usually have decent ground manners. Take private lessons. If you have to find another barn for his lessons, do it. It'll be worth it. It'll give your son some experiance, while giving him a chance to see if he's really interested to learn to ride.
-----In riding horses, we borrow freedom-----