would muzzle if you feel he is a threat to another dog, just remember he will likely be stressed as it is being boarded away from home. This could add to the likelihood that he does kick off at another dog.
Hopefully he is used to wearing a muzzle, i would recommend maybe some very tasty meat, or a piece of sausage placed into the muzzle, so he puts his snout into it. (makes it a whole lot easier)

Sometimes its better to be safe than sorry these days.

He is used to wearing one, his owner has one and it was that one I was using so he was familiar with it. This is a dog that I have seen grab a terrier before when he was younger, and the terrier was pushing its luck so not something I want to risk, walk was lovely, had muzzle off most of the time and did meet 2 lab girls that he was fine with. I felt safter knowing he was muzzled when he needed to be and his owner was more than happy for me to do it. Good walk all round