My LB is 4 months. Since learning to roll he wakes in the night, rolls on to his tummy and screams. He knows how to roll back so I really don't know what's wrong.
I would turn him back over until he starts either being happy on his tummy or rolls over by himself. Maybe some tummy time in his cot during the day to practice rolling in it?!
You can leave him on his tummy if you want to.