Dear A.,
It is difficult to determine what is going on with one developmental milestone amongst the hundreds your little one is progressing through. Rolling from prone (tummy) to supine (back) and the reverse are a part of gross motor skills which develop at varied rates from one child to the next.
With that said, according to the Hawaii Early Learning Profile chart from Vort (you can order them online) the prone to supine roll should come in between 2-5 months and the supine to prone roll should emerge between 5.5-7.8 months. At nine months there does appear to be some delay in this skill. Mind you that it is only this skill we are talking about.
What I would suggest is that you take a look at the chart I recommended, which is used by early interventionists, and see what other milestones have been met, or discuss milestones with your pediatrician in detail. If you are worried about his development you can also get an evaluation from an early intervention clinic (CSUN has one) as doing a little work earl on with baby may catch him right up. He may catch up on his own of course, but I would still do an evaluation to make sure other areas aren't impacted as well.
So don't worry as most likely he will catch up and outshine all expectations, but be proactive and like you have been and keep on top of your precious bundles development!