I'm trying to get a better sense of where you live. Is the 30 minute away the closest indoor pool? Is there a pool at another kind of gym or perhaps a hotel that might agree to a monthly contract for pool use. Is there a physical therapy clinic or rehab hospital that might have a pool? If you have not done so I would for sure get him on the list at the place 45 minutes away just in case it turns out you want it later you won't be starting from scratch. Could his orthopedist (or whoever is treating the scoliosis) make a PT recommendation or perhaps they could help him move up the water therapy list.
It sounds like you got stuck with a really bad PT experience. I had one like that too and I agree it was pointless. It sounds just the same - therapists wandering off, too many scheduled visits. However, we've had experiences that are 100% different from this too so I know it is worth trying to get something better. Just like any other profession there are good and bad. You should expect the therapist to stay with your son for at least 80% of the time he's at the clinic. You should expect to see steady progress. He should be given a home exercise program.
Is this the only clinic? If there is another clinic I would try another one. If this is the only one in your area I would arrange to speak with the director of the clinic and share what you did not like about your experience and be sure to mention you are considering talking to your insurance company about this. This is a good opportunity to model for your son that when you have problems you look for solutions and advocate for a change instead of just accepting getting ripped off. A good PT can be absolutely revolutionary for a person with hypotonia. It is worth trying to find one.
I understand your concerns about copays but with a mature and cooperative person like your son easily the therapist could arrange to see him every couple of weeks to start and then maybe just once a month and that would be radically less money than buying a hot tub or building an additional room. It would not be supervised exercise, but instead seeing that he understands exercises to do at home and monitoring when he's ready for different exercises. It is great your son tries to be active (at least before the brace) but really I can say from personal experience there is a world of difference between what a person gets from general activity than from well targeted therapy. Just as scoliosis is more common in people with hypotonia so are a range of other problems and that really needs attention. Again, great if he will exercise in water or out, but that is really a totally separate thing from specific exercises designed to address these challenges.
