How far into a feed is he doing this? Has anything else changed for him (ie has he started to roll, or similar)?

At this age, babies often wake up to their environment and he may be upset because he's finished a feed, he doesn't like the distraction, or something else. Try feeding him in a quiet, darkened room, or lying down, where you can both relax. Is he worse at some feeds than others? Are you able to bf him before he gets upset or is too hungry?

It sounds like you've done a great job up until now, and babies have a habit of changing the game just when you think you've got it all figured out.