I do hate to say this, but I think it’s fantasy land.
I know relatives (usually) mean well, but although he doesn’t intend to be, it’s almost heartless of your BiL to try to persuade you there’s some cheap and easy therapy that would have you up and walking, when you haven’t been able to for years.
There is no compelling scientific evidence for HBOT as a therapy for MS (or HSP, as the case may be). Like most alternative therapies, the best that can be said is that some people feel it has improved some symptoms.
I’ve never heard a verified story (well actually, any story) of HBOT being a miraculous cure for anyone who hadn’t been able to walk for years.
Do I think it’s worth a try? Frankly, no, if your hope is that you would walk again, as I reckon the chances are zero, and it could only end in tears.
If you had much more modest hopes that it might provide mild improvements in some symptoms, and wouldn’t be too disappointed if it didn’t, then it might be worth an experiment, but my understanding is they usually want you to commit to at least ten sessions, so even if the individual sessions are inexpensive, having to commit to a block of ten could be quite a significant outlay.
I don’t mean to sound negative - every therapy has its devotees. I guess some people must get something out of it - the same as anything else, like acupuncture, for example. But if it was achieving Biblical-style miracles, I guarantee it would have been headline news by now, and neuros would be swarming all over it.
Contrary to what some conspiracy theorists believe, I don’t think they have a vested interest in keeping us ill, and suppressing information about “cures” that already exist. I think my neuro would be as excited as me if he thought I could just toddle along to a few HBOT sessions, and problem solved! I trust him, and don’t believe he would keep it from me if he thought there was a realistic chance it might work.
I think he’d love to see a cure, and I don’t think it would put him out of a job, because neurology doesn’t begin and end with MS. If MS (or HSP) was wiped out tomorrow, we wouldn’t see hundreds of redundant neurologists. There are plenty of neurological problems apart from MS, and definitely not enough neuros to go round, so those who chose to specialise in MS might have a slight change of career direction, but they definitely wouldn’t end up unemployed.