I would just keep acting uninterested--probably not by avoiding him, because then you can never hint effectively. But when he talks to you, be polite but short and to the point and don't smile too much, and don't initiate conversations with him or look his direction without a definite purpose.

I wouldn't bring up a conversation with him about the matter at this point; you likely would make him feel like an idiot, because that's how most people feel when someone they like tells them bluntly that they're not interested. Only adress the problem if he does, first. I've found a semi-delicate way of handling that type of situation: explain that you are indeed flattered by his attentions, but that a relationship with him isn't a good idea because________ (think of some really good, irrefutable reason).