My first thought is pain- Please have a saddle fitter check his tack. I know a lot of people think they are experts on saddle fitting, but a real saddle fitter can tell you if your horse is suffering from the saddle fit. My horse was that way, resistant to going forward, did not want to work, but was great on the lunge. I had a saddle custom fit for him, so I didn’t think it was the saddle until I had my vet come out and look and sure enough, the saddle fit great when he wasn’t standing, but when he moved it pinched his shoulders.
Same thing when a young girl bought my son’s horse- I told her what bit to use on him, she didn’t listen, bought some other bit and sure enough in a week he was a different horse and she couldn’t ride him. I went out with his old bit, switched bits and when she rode him again, he was the horse that she bought.
Sounds like your horse wants to be good for you and have a real partnership
with you- please don’t give up- try the tack fitting, make sure he doesn’t hurt somewhere, and once that is checked out, then you can really be sure it is a training issue and start working him outside of the arena, trail ride, but work him while on the trail, jump him over logs, etc.