The first thing you have to do is stop referring to him/her as "liberal" because he's not. He's a leftist.
I've found that making this distinction really ticks them off.
You got it, they are Leftists - PERIOD. 'Liberals' went the way of the Ford Pinto & Chevy Nova - they're all LEFTISTS now. And the lefties who call themselves 'progressives' are Marxist Commies. (see; Nancy Pelosi)
'Liberals' believe in liberty, leftists don't. They want the gubmint to control everything and everyone. So I guess that conservatives today are truly more of a 'liberal' in that sense than those who call themselves 'Liberals' - because we believe in liberty.
And yes, calling them leftists makes them apoplectic -- they don't like the fact that we see through their little charade. So I do it every chance I get!
He gets offended when someone refers to him as a "radical". I guess in his mind it's not "radical" to want to kill as many babies as possible.