Perhaps he just wanted to remind you of how close armageddon is Gill, and used your son and his girlfriend living together to illustrate that the prophecies are coming true. Or maybe it was a sly dig at you, to imply that such a thing wouldn't have been considered if you'd still been jws.
I know when I moved in with Trev last year, we made sure the local jws heard about it, and one of the comments in the gossip mill kingdom hall was that "Linda would never have considered a move like that if she'd stayed in the truth", which is obviously right, (apart from it not being the truth of course). The point was I was now free to make normal decisions about my life without having to consult some cleaner or window washer to see if they approved, but naturally they couldn't see it that way at all.
Your dads' comment, though annoying, isn't particularly surprising. These days the more committed jws seem almost desperate to make the most of the opportunity to "give a witness", even to people like yourself, who your dad must surely realise don't want to hear it. Maybe the long time dubs, who must be really desperate for armageddon by now, need to come out with crap like that to reinforce their own beliefs, because there isn't that much evidence around these days that a lot of the prophecies they say need to happen before the big a comes are actually being fulfilled.
If he does it again, I would kick him if I were you, he deserves it.