Maybe your right side needs to be "broken in" a bit more slowly - can you express every second feed from the right side for a few days and then every third and so on - rather than doing a few in a row and ending up sore.
Are the feeds where he starts on the right noticably sorer than the ones where it's his second side (if they are then always give him the left first - and take him off after a couple of minutes if the feed "should" have started on the right)?
Is he noticeably wriggler or less cooperative on the right? Does it point in a different direction to the left??
And finally - have you tried baby led attachment, where you lean back, pop him in the general vicinity of a boob and let him attach himself??