Simple: he's an American citizen, and he has every right to vote for whatever set of individuals he wants to mold this nation into the nation he wants it to be. If that means it turns into a socialistic paradise, that's certainly his perogative.
Note: I don't agree with him, but I'm fucking sick of self-righteous tool sheds, like yourself, who don't think he has the right to live here and want this country to be whatever the fuck he wants it to be, as long as he's following the democratic fucking process. If he can convince enough Americans to agree with him, and enough amendments get passed, that's all within the fucking framework of our political system. Just because it's different than what we have now doesn't mean he can't shoot for it, even if, in all reality, he has a better chance of jumping on a trampoline to the moon.
But seriously, snide requests concerning moving to another country just make you look like an entire bag of douche. They don't help your argument, they just make you look like a cunt. For your own sake, please stop. It's the adult equivalent of saying "I'M RUBBER AND YOU'RE GLUE AND WHATEVER YOU SAY BOUNCES OFF ME AND STICKS TO YOU LOLOLOL" as a child. It's a great cop-out for the rest of the argument, because you don't have the testicular fortitude or intellectual standing to continue the argument on its own merits.