Imran has taken to sleeping on his side lately and has rolled onto his tummy a couple of times. I think that once they can roll easily tummy sleeping isn't such a SIDS risk as it is if they can't roll. Just so long as your mattress is firm not squishy and there aren't any pillows he can push his face into it should probably be ok.
You could take my sneaky approach - wait until he's well asleep and then very gently flip him over (although I only have to roll Imran from side to back not all the way over)