Romeos Klassic DJ: ...I'm only saddling him and longing him a little for now (next year I'll ride him). I think it is just him associating the cinching up of the saddle with work, because yesterday he didn't try to hurt me while I was currying him, but did while I was saddling him up.
You may have moved a little too fast in getting him to accept a saddle and also be dealing with some lingering trust issues.The heart girth and belly of a horse is a highly defensive area and needs to be thoroughly desensitized. The nipping and biting while you brush him there says that's still a weak point.
When you put the saddle on, don't go straight to girthing and cinching. Let the girth hang and give his belly area a good rub to let him know there is going to be some activity there. Then loop the billet strap through the girth D ring, raise it to make contact with his belly and then let it go slack. Do that several times before snugging it up lightly, just enough to keep the saddle from slipping if he bolts. Pull and stretch his front legs forward to make sure he doesn't have any pinched skin under the girth, walk him in a few small circles, then stop and release the girth. Give him lots of praise and then repeat the process.
You may also be over-cinching him to compensate for the ill-fitting saddle if it's too wide. A smaller saddle would be better to start him on. It has less chance of slipping and going under him if he bolts. Make sure you hobble the stirrups under his belly with a length of baling twine at first, too. It reduces the anxiety of something slapping him on his sides while he's getting used to the object on his back. And it will also contain them if the saddle slips and rotates.
Don't think that you don't need to go backward and start him under saddle all over again because he's already accepting it. You'll be creating holes in his foundation training that will come back to haunt you if you just try to push him through his reactions to the cinch. And correcting the nip and bite attempts should be addressed anytime he tries that, even if he's doing it defensively. A swift smack to his shins with the side of your boot and a firm "QUIT!" is an effective and appropriate reprimand for nipping. ~FH

"Abuse is when a human action or reaction is obviously accompanied by anger, rage or adrenaline. Proper correction and reprimand are done in silence with thoughtful intent. Your horse knows the difference." ~FloridaHorseman