My DD2 did the pinching, punching, slapping thing too so it's not just a boy thing. However, she seems to have grown out of it now - thankfully - but it nearly ended her BF journey at 8 or 9 months as it was soooo painful.

The BF necklace is a good idea. I also found with DD2 that she did it when she wasn't actually that hungry so I stretched her feeds out a bit longer and she started to feed much better with less pinching, but she was a bit older than your DS.