You can start the work on the ground. With a treat ask your horse to bend his neck on both sides and as low as he can.
Thenn when you ride him, as Andrea said, you should do a lot a lot of exercices such as sepentines, circles... Do it with impulsion and your horse on the bit with his back well placed. Ask progressively and be patient and I'm sure you will see good results very soon!
Good luck :)