Don't know all the details but any time you move up a level, there is usually some aspect of "starting from the bottom" with that new higher level. It is common to have to prove yourself all over again at the next level.
It sounds like the coach wants to reward his hard work with a move up but this doesn't necessarily mean he automatically jumps into a starting spot. There is a reason he was on the "B" team to begin with and a reason the players on the "A" team are where they are.
It also sounds like this happened at an ideal time - only a few games left. That means he got to play in a lot of games earlier at the "B" level and now he gets to move up for a bit of exposure to better opposition (???) and some good practice time with better players - those he will ultimately play V with down the road. This should be celebrated and the new challenge embraced, not frowned upon.
If son stays in the game, this is a process he will continue to face. How he handles it will likely go a long way in determining how many levels he can ultimately jump.