riding and cantering

how do i stop my horse from sticking his head down while riding and cantering?

Resist firmly but gently with the hand from his downwards movement and push on with the leg. Dont pull him in the mouth though! If you push him on more in his work with your leg he will have less time to think about putting his nose to the floor!
   
Also twiddle gently with your reins and squeeze with the leg to encourage him to drop his neck and head and work into the bit without fihgting the contact.
   
try lungeing him in side reins he mihgt be avoiding the contact by pullin the reins from your hands because he hasnt been taught to work into a contact. Let him get used to the feel by using lungeing and he should get better. it will also save your shoulders from aching too!