could he do it?

if he decides that immigration is a threat to national security, he would have the power to ban immigration via executive order, whether that's enforceable in reality or not. i still don't know how you do it, besides completely shutting the border down altogether, which would quickly lead to an economic and social catastrophe.

as was the case previously, the proper way to fight the order is to question it's efficacy, not to argue it's "racist". given that the united states has more cases and deaths than anywhere else in the world, it does not seem as though it would be a particularly rational policy. if somebody challenges it in the right way, they'd no doubt win the case. but, what we've seen from groups like the aclu recently is pretty disappointing, and i don't have a lot of confidence in them to make the right legal arguments.