hi,
I have had this too, jake managed to roll, then was not stong enough to get back or even keep his head up.
I found a great little playmat called a pat mat, which you fill with water, it has shapes in it that float in the water, and when its touched they move.
It's nice and flat so he can look at it while on his tummy.
Try putting flat rattles in sight but out of reach, to encourage her to move her arms.
I also kept rolling him back again, as it seemed to be the enjoyment of just rolling over that he liked.
It's a hard time though isn't it?As he has got stronger though, things have got better, he's happier now.
good luck x