Not everyone has them out, and not everyone has all 4 out. It depends - and not just on pain. "Most modern dentists" absolutely don't think you're nuts - it depends on the teeth, entirely!
The retainer has nothing to do with the wisdom teeth, only the teeth that are already in. The question is, are the wisdom teeth coming through straight, and is there enough room in the mouth to accommodate them? If yes, leave them alone. If no, they will undo all that orthodontia your son went through. In fact, the braces he had could have changed the position of the existing teeth, and could make wisdom teeth extraction necessary.
Your dentist can (and should) refer you to an oral surgeon based on the X-rays. You can get a second opinion if you like - that's often a good idea.
The anesthesia one gets for wisdom teeth surgery is usually not what one gets for major body surgery. And there is not necessarily any correlation with a prior brain injury and post-concussion syndrome.
Any good oral surgeon and anesthesiologist will listen to you with prior bad experiences and what specific anesthesias caused the problems in your family. There are quite a few choices for oral surgery. I had concerns too, but my son did not have a general anesthetic for his wisdom teeth although he was ver gently "under" and not conscious of what was going on.
I don't think you have enough information now to make a decision. You don't know if the extractions (1, 2, 3 or 4) are recommended, what the time line would be, what the anesthesia choices are, or anything else. You need an open mind to investigate, and your son needs to learn enough about anesthesia so that, if he should need it in the future for anything else that's not optional, he can have enough info in his medical chart to properly alert the doctors to possible reactions so those can be avoided.