How strange. What type of interaction do you and Paul have with Boomer when you all are together? When you are both up do you spend time with him? Is it gentle loving time or heavy play time? I'm thinking that whatever happens when the two of you are up together seems to make him nervous or untrusting. Are you louder when you are both together and maybe Boomer likes it quieter?
You may have to try to pinpoint what seems to upset him when you are both together. Good luck.