Well if he wasn't good enough to make the A team from the start then I doubt a few appearances on the A team will significantly change that. If the B team is of a lesser caliber of talent then your son might be a tweener. Too good for B but not quite there for A so you leave them down on B to play and get reps but at some point you need to see how he's going to fit in with the A team so you bring him up.
From your post it seems like your son is in good shape with the coaches but as RJM pointed out you say "My son and I" and you lose me. It's not about you or what you feel - it's about what your son wants and how he's fitting in with the team. You didn't mention it but what if the A 3B is having a pretty good season why in the world should he now sit just because your son got moved up? Only one person knows why your son's not playing more than what he is and it's not me or anybody else on this forum. You said "we are not allowed to discuss this" but is it really that or are you not allowed to discuss this but your son could approach the coach? I'm willing to bet if your son phrased it the right way the coach would tell him what his role would be.
You said something about him playing the OF and I've learned this in sports - I would rather be playing in another position in front of everyone on the bench instead of being the backup at my primary position.
The A/B season is what - 20 some games long????? In the grand scheme of things this isn't enough of a "problem" to get worked up over. Settle down and let your son enjoy the game and playing HS baseball. It won't hurt his development and it won't destroy his chances of getting seen for the next level.