I'd say be upfront about it and just ask. Don't accuse or act as though he's done anything wrong, because you don't know if he has, just be curious. Tonberry makes a good point, it could very well be there isn't an option for poly, or he didn't realize if there was. If thats the case, I would recommend to him that he put that he is poly in the text of his profile, that was there is no confusion. Of course, there are a couple dating sites that don't allow that at all either. If thats the case, I would recommend you point him towards a sight like OKC which has a 'taken but looking' option and seems to have a good number of poly folks.
In all honestly, if you are both new to this he probably just doesn't know how to go about things. He may be worried if he says he is dating someone, that it will scare people off, but that if they get to know him first and he then tells them, it will be more likely to work. This really isn't fair though, because you are allowing people to become attached under false pretenses. This is something I would suggest talking to him about as well. I've seen threads here discussing when to tell a potential partner you are poly, and would recommend reading over those with him. I personally believe you need to be upfront from the start, but not everyone thinks that. Find out what he thinks and discuss your (potentially differing) opinions. Decide if you need to both stick to one way of doing things or not. You may need to compromise on this, and ask that he just let the person know when they begin to show interest and before any dates or anything else, as opposed to broadcasting it initially. Its not the way I would do things, and seems to not be your preference either, but if he is making sure to tell people before things progress with them to anything romantic, that just may be his way of doing things and I think he would still be doing right by your relationship.
Also, you may want to ask him if it would be alright to talk to potential partners briefly before he begins a relationship. I think this is perfectly reasonable, because if you and them are communicating as well there is a lot less room for later problems. Even if he is completely honest and trustworthy, he can't express your feelings as well as you can, so opening the lines of communication between you and the new partner makes sure that they know about how you feel in all this, and also that you know how they feel firsthand and can try and respect and accommodate that. Its just a suggestion, but I think if you both try to put this down as an initial rule, it will make you confident that hes told his partner about polyamory, and also possibly prevent future problems from arising because everyone knows what the others expect.
I hope this helps!