BaseballMom - I agree with many of the posts here. It's up to your son to mend fences with the HS coach. Not yours. Baseball is a team sport and supporting the team (not the individual) goes a long way. It's about the team, not the individual.
As far as transferring to another HS - consult your states rules. Some states have very restrictive rules about transferring - in some cases the student has to wait one year before he can play any sport at the school he transferred to.
It's hard to say how not playing his senior year will sit with college coaches. Some put emphasis on what they saw at showcases and tournaments while others want to see how a potential recruit interacts with his HS teammates.
Was he on a showcase or travel team? How did the D1, D2 and JuCo's become aware of him?
My son's HS coach was very instrumental in getting interest for my son at the next level. He ended up at a D2 JuCo and received a nice D2 offer towards the end of his 2nd year at the JuCo.