I don't know if more exposure over time to your dh guitar playing will help or not. I've got a yorkie that is scared to death of the vacuum and has been now for 3 years. Possibly have him play for brief periods of time with Scooter in another room and see if he will get used to that, if that goes well, then move Scooter into the same room?
Good luck and do keep us updated on your success!
