So I looked into the brace thing too, and the conclusion was if you did make a brace strong enough then it would have to lock your arm in that position, i.e. you wouldn't be able to put your arm down in the plane, or pull a toggle. Basically if you want any range of motion a brace won't work.
Someone also mentioned kinesiology tape, in absolutely no uncertain terms, it is an absolute gimmick. I also looked into that, and read a handful (there are only a handful) of scientific articles that addressed its use. There were only two possible uses, one was for children, where it is strong enough to restrict movement, so basically is a brace, and the other was as a placebo, basically reminding people to protect a joint because they can feel the tape. Other than that there was absolutely zero evidence to support the idea that it could stabilize a joint or prevent injury. If you really want me to, I'll go dig up some of the articles for you.
Thems the breaks I'm afraid, PT or surgery, that is about your only hope. Left sided pull is a temporary fix and shouldn't really be your goal. There are plenty of other instances where you might dislocate it as your skydiving progress.
I agree, he should go see a therapist and be properly evaluated. Strengthening exercises are the best long term soloution
Surgery should always come after attempting all other options.
As far as the kinesio tape, I would say you are for the most part correct. We personally use McConnell taping which is somewhat different and a lesser known name. No tape or brace will fix the problem, but instead act as a "band aid".
We are able to tape up patients in a way that will limit their internal and external shoulder rotation. ( IR being most relevant for pull time in skydiving ). That limitation creates a "stability" by ensuring that the patient/athlete will not overly rotate their shoulder and increase the chance of a dislocation. So similar in many ways to a brace.
Whether or not this will benefit the OP will depend on a lot of factors ( His personal anatomy, How his container fits, etc. ) that I cant determine on the internet.
I typed this on the fly so apologize if it isn't clear or sounds clunky. These studies can sometimes be overly clinical and objective and sometimes therapist must get creative and work outside the box to help athletes achieve their goals while also helping them have a successful recovery.