If he doesn't want to nurse, then he's telling you he's full - don't make it into a struggle. Just a casual offer and if he doesn't want it, fine, try again a little later. It's not uncommon for babies to be easier to nurse at night, especially as they get older and more distracted by the world around them during the day. You're doing the right thing by taking advantage of nighttime and naptime nursing.
I never did master nursing in a sling - maybe check out the babywearing forum?