You've got 2 issues which have not been resolved fully: a head injury, and a possible reaction to medication. There is great attention being paid to head injuries, which have been swept under the rug in high school, college and professional sports. He was told by his pediatrician and the school trainer to refrain from sports, but somehow this didn't get carried over to club sports? Something fell into the cracks here.
There's also a huge problem with kids with ADD/ADHD forgetting to take their meds. Goes with the territory. He's also a typical teen in that he doesn't always tell you what's going on, and it's hard to supervise a kid of that age when they want to be independent and you still want to be the parent on top of them all the time.
I don't think you have enough answers. You can get some additional medical work if you want (for both the injury and the medication), or you can do something more natural to help his focus. My son had a significant head injury in high school (loss of consciousness too) and I also work heavily in nutrition to help with focus in kids and adults. There's SO much that's been done that is safe and not a lot of hype or dangerous self-medicating. So I've gone both routes and I think there's a balance that can be achieved.