Try a magnesium supplement (maybe consult your vet before doing so) or use draw reins.
Use a tringle system for a warm up (This is what I use for hollow or nervous horses that walk with their head up like a giraffe to help them stretch down so perhaps it will help you as well). Put your hands lower and wider (above your knee instead of wither) and gently pull one rein and another creating a pressure in his mouth like during lunging with side reins. Keep your arms nice and relaxed and reins long. If hes not giving in be a bit firmer with pulling and shorten your reins a bit. Use your inside leg to support them. Remember to be relaxed yourself to send positive energy ;) :D