Just my opinion, but you might want to share your current frustrations with your husband before acting, even on Facebook. People react so dramatically over these networking sites, I swear, so have a united front for your FIL to butt his hard head against. Because he will.
You might say, "Your father might just be throwing all his emotions into this posting, but that's long since gotten old. And frankly, I'm sick of it. He has a good job WITH the governemnt regardless of the administration, two homes, plenty of vacation time and money. What he thinks he's losing I don't know, but what he is losing is the chance to spend thanksgiving with his family, and that's his choice, because if this is how he wants to act, no one will want to host him and his bad additude. If you can convince him to check his politics at the door and stay on him to keep a lid on it, his company is tolerable if still grumpy and opinionated. If he wants to give thanks for the little miracle joining the family soon, he can play by those rules, otherwise, he's a bad influence that will just upset the baby when he starts griping like one."