Sara,
You shouldn't be so dissapointed that he won't sleep in the bouncy seat. He will become dependant on the seat and will not go to sleep any other way. I don't place my lo's on their tummy's to sleep , but only because they (twins) were premie's and there was some concern with their breathing in the beginning. I know that I have read a few posts here about mom's who do place their babes on their tummy's and have no concerns with it. I would make sure there is no loose bedding, and some people say to remove the bumber pads in the crib, if you have them. In the begginning months I never had the bumpers on. I have them back on now as the girls are 5 1/2 months old and wiggle around sooo much they actually are up against the bars in the am when they wake. Don't know if this helps.. but good luck!
Jody