If I've learned nothing else about men, women and in-laws (so to speak), it's that your partner has to stand up to their family when they do something offensive and you have to stand up to yours when they're offensive to your partner. If you try to fight the in-laws, they take your partner's silence as quiet approval of their actions.
I'd say talk to your bf and explain that if he doesn't disagree with his father, he needs to be honest about it now rather than later. Main phrase to remember: don't waste my time or yours. If you remind your partner that they lose time when they waste it lying to you, they'll remember that it's about them, too.
I'd also explain that even though he is unwilling to stand up to his father on his own behalf, you are not him, and you deserve to be defended. While it's not your job to educate his father, who is is desperate need of it, it is your job to remember that you don't need to take crap from anyone. I don't care who it is, you're too old to deal with other people's mental issues. Tell bf that there's no future for the two of you if he can't stand up for you now, because I really don't think you want to deal with it once you're married and there's more at stake than just hurt feelings. Remember, everything you see now is just a precursory glimpse into your future together. Can you stand 30 years of this?
Good luck with any steps you take...