Hi, I am sorry to hear about your son, it is a terrible burden the worry and fret of our kids and prayers for their health. It is not fair for children to suffer.
It is a very lucky and beautiful thing that the body created the colaterials. Magnificent when you think about it! But, your concern over clots is real, but also I would wonder about the scar matter or adhisions that may have also entangled the area. There are some very top excellant doctors, only a few that could exam your son and make a decision to the effects. The doctors that treat TOS are so familar with the structures, how the subclavian may have an anomolie or been moved during his surgeries or scar matter to pass un naturally through the scalense muscles in the neck. There is a animated picture of some types of TOS I like to post and will look for it.
cardio docs do that, neuro docs that, but TOS specialist see the entire picture. Is there a doctor near you in the llist? Have you had dopplar test?

With my TOS I get blood flow to the arm, but it seems blocked to flow back out. The difference between vascular and neurogenic TOS is differing and treatments differ. But the pain and symptoms vast.
Everyones TOS is different, no cookie cutter diagnx or treatments, everyones body is NOT exact, that is why tos hit so many in a different manner, injury, accidents, repetatives use, vascular, injections, etc.

The brainstorming takes place here, but real help will point to a top doc.
Hoping you find support and leads here,
dianne