I see that you have already gotten some great advice, but you can never get too much advice when you are worried about your baby! Our pediatrician said that as long as there was nothing loose in the crib (blankets, toys, bumper, etc.) that all was fine. Basically, he said "if they can roll over, they can roll back over if they have a problem!". Plus, the risk of SIDS (the reason for back sleeping) reduces greatly at 5 months. So, unless your child has specific problems.... rest easy - just like your little guy!
Please ignore the advice that said "don't ever leave your child alone, watch them nap and sleep..." that is just going to set you up for a bad sleeping future if your child always sees you there!
My 13 month old LOVES sleeping on her tummy and is how she is most comfortable!
Good luck.
blessings,
stacy