@nothernbarker If he doesn't object to others putting on his coat, but rebels when you do, perhaps you are doing something he finds uncomfortable or even painful in the way you are handling his legs? Of course, if he is anticipating discomfort and struggles, that only makes it more likely he will find the process painful. You seem convinced he isn't reacting to pain, but these things don't typically occur without reason.
I would spend some time with him, going through the process very slowly, one small step at a time, stopping whenever he objects and even going back a step, but not allowing him to do anything else until he has his coat on. Is he difficult about any other type of handling, e.g. trimming nails, checking or brushing teeth, cleaning ears, picking him up?
You say treats don't help, but how are you using them? He should only be rewarded when he does what is required. It occurs to me you could clicker train him to put his own legs through the jacket holes. Might be an interesting exercise. Hold the jacket and reward successive approximations is how I would go at it....