A.M.
Hi S.
Both my kids have slept on their stomach since the day their were born. They sleep so much better that way. My son is 2.5 and still sleeps on his tummy. I didn't worry too much about sids but was aware of it. Do what is best for your little one and your family. If he sleeps good like that and he is putting himself in that position he can get himself out of it and i wouldn't bug him or fret about it.
God Bless
A.