With a 4.5 yr old DS, I can at least commiserate.
I can also gently offer an alternative to the power stuggles you have gotten into with him. Let him be the boss and "know everything" about himself. Sometimes I find it helpful to consider the worst-case scenario for perspective.
He doesn't want to bathe -- ok. The worst thing that will happen is that he has food and dirt on his body that might get tracked into the house, wiped on the couch, etc. If he gets that dirty, explain your reasoning and then let him choose. "Oh, DS, you can't get on the couch until you wash dinner off your face and hands."
He doesn't want to eat -- ok. I know a lot of houses have different rules, ours is "this is dinner. Eat it or don't. You can always have a PB&J instead." Mine are still slightly younger, but I find that when I insert myself into their choices, it never seems to go well. Sometimes I coach, "we are about to leave the house to run errands, and if you don't eat your stomach might hurt." But no pressure.
No meds? Ok. Pain medication isn't essential. He is the one that hurts. If he starts to hurt others (screaming, fits, etc), then he is old enough to be told to keep to himself until he can control his behavior. It is his choice whether that means taking the meds, meditating, or something else.