I would agree with Clint, but also add in incremental increase in your long toss and flat ground throwing. No need to get on the mound just yet. My son plays basketball also and I know the difficulty in fitting it in to your schedule, but you must figure it out somehow.
You are only 4-6 weeks away from season start depending on where you live so get in a least 2x/wk this week and then get to 3x/week. I would recommend the Jaeger approach to LT -
www.jaegersports.com and would also get some of his bands.
BTW Clint his legs will be in pretty good shape from Bsktball, particularly since most movement is explosive. I would try to hit the weight room with compound lifts, and certainly he will need to work his core and upper body strength.
Now to answer your question. Normally I would expect a 2-3 MPH increase from preseason to in-season velocity, but you are now approaching your physical and genetic max so you will approach your max asymptotically - so like Clint suggested you may have to work really hard for some small improvements - you will not know until you get there.
BTW keep us posted as I am curious where you end up at. Good Luck!