Time, just time. My 22 year old twins, DH and some of their friends believe that while the wheelchair prevents certain things (like Mom is not coming up stairs to clean that bathroom, neither is she coming upstairs for any other reason!). We've been doing this many years. They do help when asked, but like DH only when asked. In the beginning, falls, some other stuff scared them but this is a journey your Nephew must go on before he "gets" it. Nothing happens quickly, but you'll all get there.