How old is Mack?

Please follow Canis-Lupess's advice (and please not Mrspamgoo) and find a reputable, positive reinforcement behaviorist! Just one session with a certified, educated behaviorist will open your eyes to so much. Honestly, we cannot see what is going on, so we can't give you sound advice.

My only advice until finding a behaviorist is to control the situation COMPLETELY, stop the yelling and punishing, destress and make every experience that Mack has around the Bostons (safely seperated/leashed) extremely positive. Give him high value treats every time the Bostons are around and Mack is not reacting poorly to them. Make him see them in a positive light. It could definitely be that Mack isn't feeling well, or something happened that you didn't see, or he's simply coming into maturity and going through a phase. A good behaviorist (after the Vet check up) will tell you for sure.