I would ysay do loads of counter bending and serpentines in trot, and really supple zorro up. Try to work out his stiff side, every horse has one. Simple changes should really help too. Don't do too much asking for his chnages in one session because it may stress him out at first.
Have your trainer or a friend watch you one the ground on your good sidefirst, then your bad and get them to tell you if they can see you doing something differently with your body/seat/hands/legs etc