My dad had the solution.
An elder brother once refused Mom's orders to take a bath.
Dad stripped him & put him in the tub.
That accomplished, my brother folded his arms, and said, "but you can't make me wash!"
Dad told him he didn't have to wash, and proceded to take the the big bristle scrub brush and lye soap to him. It didn't take long before there were screams, of "Stop! I'll wash! I'll wash!"
The rest of us had been observing the whole thing, just to see how it would go down. That was almost 60 years ago, and my brother hasn't stopped washing yet, and neither have the rest of us. LOL
That is what this guy needed, at about the same age my brother was.
How old was your brother then?