oh no
i wonder why he's perceiving him as a threat? it might take a while, but you could try lots of counterconditioning and positive reinforcement. starting with them in the same room, his dad sitting in a chair w/o making any noise or trying to engage with the dog, and having dexter stand far enough away that he doesn't feel threatened but can see him, with none of his stuff in there (no toys or food or anything) if thats possible and like, rewarding him for being able too see him and not growling, etc etc, for 10 minutes a day and getting closer and closer bit by bit until he's a lil more comfortable being near him and doesn't view him as a threat / associates seeing him with the love and treats. it'd take a while, but it'd hopefully help him at least a biton top of that, once he seems to have come to terms with him a little more / can be closer to and in the same room as him, having him give dexter treats and praising him whenever the dad walks into a room that dexter's in and he doesn't growl, etc will help him start to associate him with good things, and don't forget to use a leash and possibly a muzzle during training jic
i am not a professional trainer tho and it might help to find a behaviorist or a trainer who can help you, tho i know that could also be a bit costly and it can be hard to find one who's certified and trustworthy.
i know how frustrating behavioral issues can be, i hope you're able to help him so that he and your bf's dad can feel a little saferedit: sorry for rambling--
