I would go to see an orthopedic specialist and get a specialist opinion before leaping for the MRI expense. Regular vets are not equipped to diagnose potentially orthopedic problems and it is worth getting this opinion first.
Berners frequently have back and neck problems - while severe disc issues are not common, vertebra compression is and worth ruling out. It sounds like he could have anything from a C4 to a C6 out of alignment, or as the vet suggested and I agree is more likely, there is also nerve involvement (given the signals to the front leg).
If you were to consult a physical therapist, they would explain where the nerve endings are located and how something out of alignment in the neck can compress, and readily send nerve signals down the front leg and appear to be a leg issue.
Berners are heavy boned dogs so if they are big jumpers, or hard wrestlers with other dogs, they readily get themselves out of alignment in their neck and back. Also, if a dog runs head first, then turns his head to brace himself and hits something with his neck and shoulder - this is easily done as well and a common action for a Berner.
Hopefully that is all this is and some visits to a physical therpist can get him re-aligned and back in order.
Find a Board certified orthopedic specialist and start there. Hopefully they can refer you to good physical therapists in your area if that is appropriate. Where are you located? Have you spoken to your breeder?
