Give him something to play with while breastfeeding. Rubber toy, muslin cloth, soft toy. Find something that he would like, and every time, tell him "no", remove his hand, and give him something to fiddle with.
I'd unlatch him, say 'no' in a firm voice, place him down on his own and give him a teether so he learns he can only bite the teether not mum
Thanks ladies - that’s really helpful!
I would unlatch him and say a firm but gentle “no”. If he does it again then I’d put him down. They soon learn that it’s not the fun reaction they’re after! Breastfeeding necklaces are great for twiddlers or to offer as a teether when they’re in a bitey mood.