DD slept on her tummy from about 2 weeks old, she was just more comfy adn slept better that way!
I never had anyproblems with it, obviously taking the proper precautions - ie sleeping on a firm surface, not on rugs and such and making sure that she can breathe freely etc - and he will be fin
Having said that, I was addicted to the monitor and checked her frequently...once they can move their head they are usually alright

Good luck!