My son played soccer last fall and he definetely needed the meds for games. We forgot meds one morning and and he couldn't keep track of what was happening, or even what position he was supposed to be playing.
This year he wants to play football which should be a totally different experience. The youth soccer league in our area is very relaxed (1 practice, 1 game/week), but the youth football league is more competitive and practices 5 days a week until school starts, then it's 2 practices and a game- not to mention that the game is more complicated. It should be interesting to see how it goes.
I don't think there are any additional risks when taking meds- if anything I think there'd be less risk of injury because the person will be more focused and better able to keep track of what's going on around him.
I assume you've checked in with his doctor--she might give you and him advice on what to be midfull of or to watch for. When I first started taking stimulant medication, I noticed various side effects related to physical activity (I'm an active 50 year old, mind you, not a HS xcountry runner!). On a couple occaisions I had dizzy spells after bending over weeding for a prolonged period in my garden, then standing up--woooozzzzeeeee!!! Another time, maybe 6 weeks after I started taking meds, I went on a club bike ride and I rode fine, no real change in how I felt, until, that is, at the end when I got off the bike and nearly fainted! No warning, no nuthin! I didn't faint but came damn close. All this dizzyness stuff is long gone now. I used to avoid exercising while a full dose of stims was coursing through my system...perferring to wait until the dose was tailing off. Doing so just felt better to me whether I was running or biking. But now it really doesn't matter as I don't feel much difference.