As the others said, it would be best to tell him, of course, and early on. He has a right to know what he's getting into. But if you really like this guy a lot and really don't want to lose him over this, let him know that as well.
Also, if you really, really do like him a lot, you could promise him that you'd be exclusive with him for a given period of time -- a year, say. If he'd like that, or if it would help him to cope with your polyamrous inclinations.
Oftentimes councelors and relationship advisors say that people can transition happily into poly- down the road, after having established a good strong bond and worked out how to communicate well with one another, etc.--, but that trying to do it too early in a committed relationship can interfere with the building of trust and bonding. I suspect that this can be true for some people, and that it could be true of the guy you're seeing. Of course, when two already experienced or capable poly folk form a relationship, things are different and they may not have any inclination or need to wait, or may already have other partner/s.
Again, if you really like this guy and don't want to lose him over your polyamory, LET HIM KNOW THIS. He may take a while to comprehend the situation and decide if he can deal with it. Or he may already be up to speed; you never know.