Hi! Just to echo what others have said...if he's truly following his adjusted age developmentally, I wouldn't be too worried if he's not fulling rolling over yet. Just practice, practice, practice with him...every day, every A time, for a few minutes at a time (he'll let you know when he's had enough). My DS hated being on his tummy, so getting him to be on it long enough to achieve rolling was not fun
He was about 5.5mo when he finally did it; walking assisted by 9mo; walking fully on his own by 1yo. He hated crawling and only did it when he absolutely had to.
You can also practice with him on pushing with his feet by putting him on his tummy and putting your hands on the soles of his feet to give him something to push against. I also think a jumper is a good idea. My DS loved! his. I'm not sure if a walker is safe for him just yet. You can get a jumper (so, a stationary toy) for $50-70 new. Less if used.
HTH! Just keep working with him