If you otherwise like and enjoy this horse, I think it's worth exploring whether spending some time with another, more assertive rider won't get him out of the habit of testing you every ride. I think you are very smart to recognize the problem for what it is, and to question if you want to continue dealing with it.
If the more assertive rider doesn't change the behavior, or the horse reverts to the old behavior shortly after you start riding again, then you may have to consider selling him and finding something else.
Truthfully, I doubt your feeding regimen is the issue. However, if you can change/increase his turnout, I would certainly try that. Increased turnout improves a *lot* of behavior problems.
I have a horse almost exactly as you describe. I call him a "riding school dropout." You don't have to be a particularly skilled or sophisticated rider to to ride him, but you do have to be willing to deal forcefully with his "You can't make me, nyah, nyah!" moments. I never put anyone else on him for that reason. When I come back to riding after surgery shortly, I'm going to start back on my guest horse - I just don't want to be in a situation where I let the "dropout" win that "make me" battle because I'm not physically up to it.