Just take a step back and evaluate. Politics aren't worth losing family over. Calling him out would do nothing but make both of you angrier. Every member of my family was raised in the south. My parents were the only ones to "get out" of the alcoholism and move north; thankfully I was raised with northern values. However, some of my great aunts/uncles and cousins are disgustingly either racist, sexist, or islamophobic. It's terrible, but I put up with it becuase I love them. I may nudge them in the right direction when I see them (by making a discouraging comment or drawing a line and saying, "Please not in my house"), and they usually respect that adn apologize for their out-of-place comment(s), but at the end of the day they still believe it. This is kind of different because it's behind a computer screen. I'm sure, to reiterate what you said, that your uncle doesn't go on crazy, islamophobic rants very often to other faces, but he is experiencing the phenomena that everyone feels- empowerment behind a computer screen. I don't know if there is any "ignore" function on Facebook, as I'm not on it, but if there is any way to not see his posts with no negative connotation (such as "unfriend," "unfollow," or "block") I would do that. It's not necessarily his behavior, or even his personality, that bothers you- because he's not like that in person and he probably never mentions it in everyday conversation- it is his online persona. I'd act accordingly.
At the end of the day, everything "belongs" on the Internet (there are no established or widely-accepted boundaries regarding what to say or how to act), so I try to not take what people post/say there at face value. It brings out the worst in some people, while in reality the phobias, mean-spirited thoughts, and false bravado are probably less than 1% of your uncle's actual personality.