Is your bit fitted too low? This would make it loose enough to be pulled through his mouth. Also you could try loosely fitting a flash or drop noseband so that he can't open his mouth wide enough to let the sides of the bit in.
Are you sure you weren't pulling the bit through? This problem is often caused by the rider (sorry
). If you're sure it isn't you then it may be worth getting his teeth checked as sometimes they'll manouevre the bit to avoid a sore area.