http://www.hognose.com/pages/care/east.htm
That's a very nice care sheet for them that will probably answer many questions for you.
About the venom issue. I've seen quite a few people bitten by heterodon hognose snakes and the most I have seen come of it was a little itching. The only thing I would be at all concerned about is if he is super sensitive to things like bee stings. If he is, then a bite may cause a problem...then again, it may not. Personally if I had a son who was 8 I would have no problem letting him own a hognose snake.
There is a lot of research going on and that has been done as far as how to classify hognose snakes. You'll hear from one guy that they are venomous with rear fangs and all. You'll hear from the next guy that he was all wrong and that actually they have no venom whatsoever. Then you will hear from the next guy that they don't have anyvenom or fangs, but just a slighty more irritating saliva than most snakes. Then you will hear from the next guy that everyone is wrong and that have venom glands and all other sorts of things. Everyone has heard something and hears other things. I am willing to bet the person at the pet store you spoke with heard somewhere that hognoses are venomous and just took it at face value.
So in the end are hognoses really venomous? A case can be made that they are. Are they venomous like a copperhead or rattlesnake? No way, not even close. Are they a risk? I'd say no. I've had hoggies for years and never had any reaction to a bite, and bites from them were few and far between.