C.P.
At the beginning with my little one, he would only latch on to the left breast so I would pump the right breast after each feeding. It took a few weeks, but now he feeds off of both breasts.
My neighbor fed exclusively off of one breast for five months before her daughter would take from both breasts.
If you breast is so sore, perhaps you should take some time off from using it until it heals. Once it is healed, maybe you could try him back on that breast and you won't have the pain.
Also, have you seen a lactation consultant or consulted with LaLeche? They may have some suggestions for you after watching you feed him on that side. It took my son forever to get the right position on the right breast. I broke out with numerous LARGE blisters and bled until he correctly latched on. I found the football hold was the easiest with him on that side.