Welcome to NT!

Yes, the things you're describing (trouble concentrating, fatigue, impulsiveness, etc...) are all classic symptoms of PCS. Frankly, after multiple concussions, I'd be shocked if your son didn't have PCS.

Has your son had a neuro-psychological exam? The exam is an absolute must. Firstly, it will tell your son how his brain is actually functioning. (Standard CTs and MRIs are not sensitive enough to see the torn and damaged axons. Often people with obvious cognitive deficits are told their scans are "clear.") Knowing how his brain is operating is the first step in getting treatment like cognitive therapy. A confirmed diagnosis would also get him gain access to help at school be it extra time, tutors, etc...

The Brain Injury Association of America www.biausa.org is an excellent resource for patients and family members trying to understand and treat this sometimes life-altering injury. Another site to check out is www.tbiguide.com

PCS is tough: I extend all my best wishes to your boy.

Cheers