Hi, I don't feel like an expert, but I have kids and a Starsurfer M. My younger girl is 9 and 25 kg.
For your son, the M will be very stable again, kind of a barge too. However, it is much shorter and lighter than your other board. This will add a lot of maneuvrability and I am certain your son will like it.
The small might be better in fact, but it is really a question of skill. I had my kids try a 87l FSW board, so this is what I can compare too. This will float your son very well, and move fast and easy. However, lateral stability is not as good and my girl fell because of that. She didn't mind at first, but after a few times, she managed to scratch her knee on the board and doesn't want to use it anymore.
I guess that if your son is totally confident with handling the sail, he might be ready for more challenge. If not, don't push it too hard.
Also, the sail sizes are critical. When my girl is comfortable with a 2.0, she might not be able to handle the 2.5. The reason is simple, for me it would be changing from a 8.0 to a 10.0.