SupernannyA, I always found it a bit difficult to change the nappies of friends' babies, actually. It was just different, because it wasn't my own child. However, this wee boy is yours. Does his mum/dad know you have a bit of a problem with this?
The wee lad wants to come and stay with you, because he must love being with you! Try to think of the dirty nappy simply as something that has to be dealt with quickly, because he'll be uncomfortable and sore if it isn't done. Get some latex gloves, a pack of nappy-sacks etc and just blether away to your grandson as you tidy him up. 