I always stretch my horse's neck out as soon as I get on..before you start moving, pull their head to the side both ways (hopefully on a soft rein, I don't know how to say that more gently!) . Don't release until they are soft and not pulling against your reins. The ear closest to you should flick towards you, saying "my attention is on you." Release when this happens, and I usually do a couple times both ways and call it good.
You can also work on just lots of bending exercises. Trot around barrels, do serpentines through some poles, tight circles, etc...Do it all at a trot, or work up to a lope to give him more of a work out :) The more you ride, the better and more in-shape he'll get.