First of all, make sur your horse doesn't have any pain on this side.
Then, try to work on the ground to make him bend his neck on both side (a treat can help!). Do this exercise each time before riding.
Then, when riding, give him a good warm up with his head low and without asking too much. When he is ready to work, do a lot of circles, figure of 8, serpentines and all the exercises that make him bend. Do this on both sides even if there is a weaker side.
At the end of the session, let him stretch out.
This will take time but if you are patient and listen to your horse, I'm sure you will succeed!