Riniel Jasmina wrote:
I'm also playing around with making it so he can't hear them or making it so he just gets a certain feeling that tells him they're near.  The problem is, that raises questions of how he knows they're evil or even really there.
I think this is a great idea! I've kind of jumped on it and mentally developed it a bit. Here are the thoughts I had in response, for your consideration:
I think those "problems" could be turned into plot developments. Suppose, at first, he merely sees an unusual sort of shadow or darkness around or beside people, and doesn't understand what it is--only that it gives him a deep uneasiness. He could learn they're evil by association--he sees the shadows "enter" a person (or sees wisps of them seem to drift into someone's ears or eyes) from time to time, frequently when they are faced with a moral decision. Perhaps he could identify a time where the person changes their mind and pursues an evil course of action after the shadow's "suggestion". [narrative]And then he paused. 
When did I start thinking of the shadows as making "suggestions"? He shook his head to clear his thoughts, but the notion rang true with uncomfortable accuracy.[/narrative]
As he reads scripture (or hears a sermon or something) he makes the connection that he is somehow seeing demons, and perhaps 
then he finds he can begin to predict what some of the suggestions are when he sees them happening. (Perhaps you could even leave it unclear as to whether this is a supernatural/prophetic gifting, or perhaps just an intelligent, discerning mind, armed with the extra sense of knowing exactly 
when a demon is actively suggesting something.) Perhaps, later on (by what grace he gets there is up to you), he would see the shadows as if in clearer focus, and be able to make out what the demons look like--their appearance could perhaps mirror their intentions for the person to whom they attend.