And we both missed it!!
I was upstairs brushing my hair, getting ready to go out. H was downstairs with J, but he had his back turned for a moment. When he turned back around, J had rolled completely onto his tummy. I made it downstairs just in time to see J on his tummy and to find him rolling back onto his back.
We tried, but couldn't get J interested in trying to roll over onto his tummy again. I can't believe we missed it. I am so sad!