How did your lesson go? I'm thinking this horse has become arena sour, doing same old, same old. If you're not trail riding, do so. You can still school on the trail but he may just need a relaxing ride with another horse and rider along. Why do you lunge him and for how long? The idea behind lungeing is it's a means to push your horse around, tell him what to do. A few circles usually suffice at each gait or if he does something better then reward him by allowing him to stop (step back but don't pull on the line). Much more can often be accomplished in brief sessions than by plugging away at something.
I had a friend that when her horse went faster than a trot, she would lean back on the saddle to keep her seat and she always had a problem with her horse stopping in the lope, we didn't realize why until we really looked at her riding. Maybe he's sensitive so your seat and your not staying centered in the saddle?