I would urge him not to quit during the season. That burns a bridge. In future years, that coach may retire, or your son will grow older and be eligible for teams coached by others. If as you say the problem is the coach, it is a problem that will pass in time. But quitting will label your son as a quitter, and that will keep him from ever surviving tryout cuts in the future.
As for summer and fall, not bad ideas, but I would consider whether maybe your son needs some time off. 14/15 is the age where lots of boys decide that baseball isn't the most important thing for them any more. Maybe your son is one of those boys. Sometimes parents call this "burn out" but often it's nothing more than the kids recognizing that they've hit their ceiling and it's time to move on to more productive pursuits.
Also, ask someone who has seen the team, and whose opinion you respect, to give it to you straight as far as assessing your boy. You wouldn't be the first parent to suffer from rose colored glasses syndrome. Get yourself some objective information. Maybe the coach is right all along.
As for size, time may well take care of that. Lots of kids grow quite a bit from age 14 on. If that's the issue, then he could stick with baseball and continue developing his skills in the hopes that the genes might yet kick in and give him more size and strength to work with.
Whether coach is right or wrong, playing time is an area where you may not properly question the coach's decisions. You need to steer clear of any attempt to discuss such things. The best your son can do is stick with it, work hard to improve, and maybe when he gets to varsity age he'll get a fresh look from a different coach.
If your son agrees to try that path, then summer play (for coaches who appreciate him, who will play him and offer sound instruction) can only help.