Well, it sounds like you're in for a fun family reunion. You could always try pretending you're in a spy movie and you have to avoid your father at all costs. Or, you could go the more traditional route and just avoid eye contact and pretend he's a ghost. On the...
Well, if you really want to be more assertive, the first step is to not ask for advice from a life coach/appliance repairman. I mean, come on, buddy, we fix dishwashers and give pep talks, not turn you into a confrontational badass. But since you asked, have you tried...