J.W.
No worries. My daughter did the same. She HATED being on her tummy so if I put her there, she would roll to her back and stay. She's perfectly fine and happy!
If he doesn't like tummy time like mine, you can try rolling up a receiving blanket and putting it under his arms. It gave her enough space b/t her and the floor for her to tolerate it and she learned to roll that way. I don't remember when she rolled though.
J.