Hmmm, since undressing is the point of contention, how about undressing him during the shower, instead of before?
Just a thought. Might help, might not.
I used to wrap my grandson in a towel, and lower him into the warm bathwater while he was still wrapped up.
Now that my DIL’s bathing him, she wears a bathing suit and gets into the tub with him. Not sure when in the process his clothes come off.