He could also just be telling you he doesn’t want to be working so much or jump anymore.
Like everyone said, I would check saddle fit. I went through 3 saddles with my 18 year old due to him not being worked, then being worked, losing weight/muscle and then regaining muscle. It’s a never ending cycle with the older ones because they are constantly changing.
Are you kicking, carrying a crop?
Who was riding him before you bought him? Have you talked to them about this, to see if maybe this is why he wasn’t ridden so often?
I have one gelding who if you carry a crop, freaks out and bucks(he was whipped by previous owners).
so saddle fit, teeth, feet, joints, etc. Maybe take him out on a trail ride, get him out of the arena, see if this helps his attitude.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."