Gloria, you know that I've had more experience with gallbladder mucoceles than anyone wants. If I've learned anything after two surgeries and another one in treatment for GB, it's the importance of scanning every single Sheltie by the age of 6.
You were so smart to have Beau checked now, when he can have the surgery before he's in crisis and the odds of survival in surgery are slim. The odds of survival and successful recovery for gallbladder removal before the GB ruptures are far higher, and it's so much easier on the dog...and your checkbook. The cost of surgery nearly doubles when the dog is an emergency, although it's expensive enough even if they aren't.
As I've said in other threads, Lacey had her gallbladder removed at age 9 after numerous scans indicating sludge in her GB was increasing. She had no symptoms. Checkers had hers removed at 11, for the same reason, also no symptoms. They both recovered in days, came out bright and lively, and eat a totally normal diet. I'm praying the same will be true for Beau. He's young and healthy, and will do just fine, and you'll have many more years with him.