I would keep him isolated. BUT, in my personal opinion (and my opinion only) I see no harm is short visits so the cats can start to get used to him. They aren't going to catch FeLV in a few short visits. One of the things you would want to prevent would be drinking from the same bowl, But your cats won't be all cozy and rubbing up to him at first anyway. But if you are going to worry, than just keep him 100% separated and he will be find. That month will pass quickly.