First off, throw the abuse out the window. Horses don't understand pity and a strong horse will take it as a weakness on your part. Treat him like you would treat any other horse. Just take it a step slower.
Why wont he allow you to cinch him up? Does he bite...kick...rear? Try doing it in small doses. First allow the cinch to touch his belly. If he tolerates that, give him praise and move on. If you are riding western, allow your latigo to go through the cinch and let it rest on his belly. If he has been trail ridden before, this shouldnt really be a huge issue.
What would you like to do with him? If he walks, trots and canters in both directions and knows his cues on the ground...he probably knows them under saddle. Teaching him to move away from pressure is a good place to start.