70-75 pitches in a half-hour bullpen setting is, I think, a bit much. Think about it. 70-75 pitches might be what he throws over the course of a game. But, most likely, that game takes a couple hours and he gets to rest every other half-inning while his team bats. FWIW, Tom House recommends bullpens of about 40-45 pitches.
If I'm reading this right, your son throws a bullpen on Monday and then has regular practices on Tuesday and Thursday. You mentioned limiting outfield throws in the Tuesday practices - can I assume he does no pitching in the Tuesday practices? If not, then that's an obvious problem.
My understanding is that swinging a bat is a joint loosening activity for the front shoulder as the follow-through opens up the front side of the shoulder. So, yes, players who pitch left or right handed and bat right or left handed are at greater risk for injury than those who pitch and bat with the same "handedness". But the concern here would be the shoulder and your son is having pain in the bicep. I guess I should ask where in the bicep the pain is located.
One thing I would like to mention is that of periodization which is basically shutting down for a period of time. The American Sport Medicine Institute (ASMI) has issued recommendations on this. Go
and click on the link for "2006 USA Baseball Guidelines". Some folks say that pitchers should shut down their pitching for a certain period each year but that they should continue throwing. The ASMI, however, recommends no throwing at all. I think the difference has to do with age. I feel that year round throwing (throwing - not pitching!) may be ok for older kids but not for young kids. Too often people make blanket claims or recommendations without qualifying them appropriately.
There is also a link there for "2007 ASMI Presentation to Little League". Click on that link and then on the link for Section 3 which is a presentation by Dr. James Andrews. The presentation is very informative and I highly recommend watching it.