My son did the exact same thing... could roll from back to tummy during the day and at night. But would only roll from tummy to back during the day. If he woke up at night on his tummy, he would scream.
We let him cry it out... first night, 45 minutes. Second night, 15 minutes. Third night, back to sleeping through the night (meaning 9-11 hours!).
He will figure it out. You just have to decide how quickly you want him to do so. If you are okay with the letting him cry it out theory, try it! It worked great for me! And he'll quickly figure out that it's okay to wake up on his tummy and go back to sleep all by himself.
By the way - my son is now 7 months old - and he'll roll from his tummy to his back in the night all by himself.
They are smarter than we think!