That happened to Joel when he had earmites, we had to lay him on his side and hold him down to put meds in his ears so then he didn't like to be put on his side for brushing or whatever. But to get him used to it I would lay him on his side just to pet and cuddle him and he's getting better with practice.
So maybe you could try that with the sling, put him in it and carry him to the kitchen and give him something good like meat
