How sad that some people want to string along a guy that is clearly falling for the op and not clear things up. You don't owe the guy you are currently bedding a relationship or an conversation about your thoughts being with the possibility of reconciling with an ex but if you know for a fact that he is developing feelings for you then you need to do the right thing which is have a conversation with him again and ask him if he is indeed catching feelings when your deal was "nothing serious."
Someone with integrity would have that conversation. Someone who was confident and knows that she is a good catch that will catch someone new if she should lose lover boy and things with the ex don't work out, would talk to the guy and let the chips fall where they may. Anything less is just plain shitty and self-serving.
Op: I think you want to tell him hence you started this thread... If you didn't feel you needed to talk to him about it then you wouldn't bother asking us... you'd just go about stringing him along as backup as you pursue a possibility with your ex.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion