It probably feels different when your brushing him with it.
sometimes my pinslicker brush makes a funny noise as well as it goes through thicker coat or coat with knots.
Little steps, show him the brush and maybe run it over his body but not the actual brush side, praising all the time. Slowly turn the brush over and run it gently over him with the brush side.
Sometimes it helps if you brush them on a table or standing on something thats off the floor so that they realise it's not a game, and learn to associate being brushed with that etc.
Hazel was scared of the slicker at first, I do think it's the feeling of the pins, as it's probably weird at first, and she still doesn't like it on her leg feathering.