Advice here in NZ is to put them to sleep on their backs to reduce risk of SIDS. This is mainly with small babies. However both my two once they could roll would sleep on their tummy's, sides, squashed against the side or end of cot. My daughter is now 8 and still sleeps in some weird positions.
I would put them into bed and then once they were asleep cover them with a blanket. In winter I would also make sure there was a heater on in their room as they wouldn't stay under their blankets and put them in sleep suit.
It is possible that if he had a cold when he started this that it was the most comfortable position to sleep. when they were small and had colds I would often prop up the end of the mattress with a pillow under it so that they weren't complete asleep.
Essentially at 1 I wouldn't worry to much if he is sleeping well in that position.
SAHM to two school kids A (9.5) and R (7.5)