When I had this problem, several LCs (including my mom, who I trust above everything) suggested to try feeding on the unaffected side and pumping on the sore side for several feedings, to give it a break and a chance to heal.
I had gotten so torn up at the beginning that I eventually had to use a nipple shield for about 8 weeks, starting at around a month. Not all babies will nurse with the shield right away, and I wouldn't recommend your using it just yet as he is still a little young for artificial nipples of any kind. It's something to keep in mind, though, as a temporary means to keep him feeding on that side while reducing the friction and soreness.
I had gotten so torn up at the beginning that I eventually had to use a nipple shield for about 8 weeks, starting at around a month. Not all babies will nurse with the shield right away, and I wouldn't recommend your using it just yet as he is still a little young for artificial nipples of any kind. It's something to keep in mind, though, as a temporary means to keep him feeding on that side while reducing the friction and soreness.