The issue with initial training in a motor glider is the complexity. There are a lot of system to learn and then to learn how to deal with the failure modes of those systems. It would be kind of like trying to learn how to fly both the towplane and the glider as opposed to just learning to fly one or the other.

Club operations generally are great for low cost at the price of time commitment. The social aspect is also a bit benefit of clubs.

Commercial operations are much better for learning with the least time commitment, but of course cost more.
Many operations are very good at accelerated training where you can stay there for a couple weeks and complete the training. But do your homework and get some recommendations.

A hybrid approach would be to start your son with some power training, which probably is available locally. The Age limit only applies to soloing, not to receiving training. But especially if he got 15-20 hours in a tailwheel airplane (tri-gear would be ok), would get him to where he was about ready to solo and he would develop most of the skills needed to fly a glider. Then spend a week or so at a commercial operation to get his glider rating.

Brian
CFIIG/ASEL