Still new to the game--couple months(one season). So if a rookies' initial max vol goes up 8 points and his current goes up 15 can he still gain on both vols his rookie year? For that matter can he gain more max vol during his 2nd training camp? And so on?

Yes, as long as the actual number is less than the potential number, the potential number can keep going up. The first training camp typically see the biggest movement, but that’s not always true.

A simple way to ballpark how much the potential is going to go up is take potential - actual and multiple the difference by the Vol/100. For example:

Vol: 60
Pass Catching: 15/60
Difference: 45
45*.60 = 27
60+27 = 87

So this players pass catching would end up around 87. There is random variations that make it impossible to predict the ultimate value.

What Mardn said plus your HC and coordinators depending on their attributes will help speed up the progress. Just slightly.