Again-another spoiled horse. I have a feeling, since you say you cannot move him even on the ground, that you are asking not TELLING him to move. It is the difference between "Pweessse move nice howseeee?????" And "MOVE YOUR BUTT-NOW!" When "asking" him to move, simply ask, tell, DEMAND. I ask by raising my hand and pointing in the direction I want my guy to go, with the line in that hand. I cluck for trot, kiss for canter. If he does not respond to that, I ask again, this time moving the whip toward his butt. If he STILL does not move, I TELL him, by the hand, sound, but this time I also take a step with one foot toward his butt, and MAKE CONTACT with his butt with the whip. Make sure you are making the contact. I will be he will move.
Under saddle-same principle. Ask, gently, then more firmly, then move to using your reins, a dressage whip or crop to MAKE him move his butt. But as has been said, make sure you "leave the front door open" for him to go through by leaving his mouth alone.