Try a glove - I got a cactus one, sounds harsh, but he liked it, because it felt like a hand, not a brush - it had a 'polishing' cloth on the other side of it. He would throw fits with any sort of body brush, even my 50 years old if it is day super soft one, but tolerated the mitt.
i've got one of those soft polishing mitts, it is frog green with really soft long strands like a shagpile carpet, it looks like a muppet's scalp, and my mare loves being groomed with it! i think it came from Halfords or B&Q or similar!
Just remembered i have a soft brush made by equerry, its called a finishing brush. It looks like a dandy brush, wood back with black bristles. I think its horsehair. Just checked and it has the code LDHH on it
My mare hates having her face brushed so I got a leather backed stubben body brush which has really soft bristles - not sure what they are but they even feel really soft on my skin !!
Won't dare wash it though so kept for cleanish face only (it loves to make mud pies with it's head !!) so chip off mud first (usually rub with hand, etc.
I've got one of the Salmon goat hair body brushes. TBH it's SO soft it's hardly worth using!!! I prefer using a piece of velvet cloth. It achieves the same result and is less work.