When you say too large....in what way is it too large? Is the gullet too wide for him, or is the actual seat of the saddle too long? A too big saddle can do just as much harm as a too small one. What's your clearance in front? IE: When you're riding and you stick your fingers between the wither and the pommel, you should have at least 2 to 3 fingers of space.
But yeah, as I said, try him in another bit, something with an elliptical or french link, to see if he's any better.
It is not 'cool' to ride without a helmet! period.