I was also about to post and say that it sounds more like 3 3-hour sessions a week, given the name of the group. That would be slightly more reasonable - over £100 per month for 3 hours a week would be ridiculous.
I have two DC doing gym - one competing at elite level, and one just starting to compete. The older one was doing 15 hours per week for £140 per month. He has now moved clubs and is doing 16 hours for £110 per month. The younger one does 3 hours per week for £60 per month - so a big difference per hour cost.
In both clubs, the cost per hour reduces as they add more hours. All the squad sessions are cheaper per hour than the rec classes, as no-one would be able to afford for their children to keep going if they were paying rec class rates multiplied by anything between 15 to 26 hours per week. The rec classes in most clubs subsidise the squads.
Yours is on the pricey side, but as long as the cost per hour comes down as he professes, it's not outrageous. I'd be more concerned about him going straight in from essentially nothing to 9 hours per week. DS1 was doing 8 per week at age 7, but he'd built up over 3 years, rather than jumping straight in.
If he's keen to do gym, I'd be looking at any other local clubs before committing to this one.