I would definitely get him checked out. Please don't rush out and buy a flash straight away, i've seen so many riders put flashes on their horses just for the sake of it. It just turns into a vicious circle because 9 times out of 10, there is physical problem with the horse, so by strapping it's mouth shut it just makes the problem worse, but if it is a training problem, all the flash does is cover the problem up and it doesn't solve it. There is no substitute for good schooling is what I think :) . But yeah I would definitely get him checked out, hope this helps.
Hoe you get it sorted x x x