My DS is 3 months and usually when feeding scratches, pummels and grasps at my breasts. It is not nice! I interpreted it as wind making him uncomfortable but winding him doesn't make a difference. I swap breasts which helps for a bit but then he starts again. He even manages to get to them under a pulled down tshirt! Is it a supply issue? He's putting on weight fine and has just started sleeping 11 hours through the night.
DD went through this stage and has recently also started tapping/slapping. I use a pillow to feed her on and tucked her lower arm under my arm and held her other hand. A teddy also worked. You have my sympathy though as I seriously looked abused for quite a while with bruises and scratches all over my arms and breasts.
You could try a nursing necklace... My DS grabs and squeezes (always has done) I either hold his hand or wear a necklace that he can play with. If you google nursing necklaces you can get one that's 'baby safe'
What about when they pinch your ARM so hard it leaves bruises? DS3 has such a...painful...pinch. Yet it doesn't really hurt me when he pinches my boobs, but MY GOD when he pinches my bingowings delicate under-arm area!!