Can Jehovah's Witnesses date or marry other Christians?

I don't think this would work. Do you know any JW doctrine? A lot of it is the complete opposite from Catholicism. Then if you do get serious with this, you will both likely want to have the other to convert to the other's religion (he'll want you to be a JW and you'll want him to be Catholic). Plus, if you have kids, what religion are they going to be? I'm not trying to dissuade you from being with him, but this is an issue that is going to be around unless one of you converts to the others religion.