I would personally consider selling him. Horses are suppose to be fun. We ride because we love it and we love with with the horses. I myself have been in sort of a similar situation with a horse that was beyond my experience at the time. I kept telling myself he would get better and I could do this, but I was barely riding. And it wasn't fun anymore. That was the drawing point for me, when it became less fun and more of a hassle and stress to go out there and work with that stubborn horse, while we didn't get along anyway. It was a bad decision to get him in the first place, but I was horse-less and he was free.
I think training is a good idea, but I'd also not let go fully of the idea of selling him. It sounds like he just needs someone who is a good match for him training-wise who can teach him right from wrong, but if this one issue is solved, what about anything else he might throw at you? Thinking past one issue to the whole picture is what helped me. I thought I loved my horse, but I loved the idea of him more than him himself, as I didn't enjoy being with him.
I hope you are happy with whatever choice you make.
~He knows when you're happy
~He knows when you're comfortable
~And he always knows when you have carrots.