This may sound simplistic, but try dosing him from
the other side, if you haven't already. Believe it or not, it works on
most difficult horses. I've had horses that were near impossible to dose
from the left not even raise their heads when dosed from the right. Stand
on his right side facing the same direction he is, with your left arm under his
jaw and your left hand resting across the bridge of his nose, and slip your
syringe or whatever into the right side his mouth with your right hand.
You have more immediate control of the horse with your hand across the bridge of
his nose than holding the halter in case he does object.
Try it and let me know if it works with your
horse.
