What kind of phone is it? Is it password protected? Have you asked to see the text messages? Can you ever get 10-15 minutes of uninterrupted time with his phone? (you have to have the password if he locks it)
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Do you know who the Other Woman (OW) is? Is the OW married, or in a relationship? If you can find out who she is, and then expose the EA (emotional affair) to her partner, it will go a long way to helping you deal with this issue.
You must have hard irrefutable proof before you confront him further or attempt to expose the affair to anyone. Have you looked at the phone longs? Does he call her as well as text? If he calls her too, you need to go purchase several Voice Activated Recorders (VARs) they can be purchased at any office supply store, on-line, or at big retailers like Walmart, for under $50 each. Velcro one of the VARs under the drivers seat of his car, review what it records. You can replace the VAR with one of the others while you are reviewing it for inappropriate conversations.
Install monitoring software on any and all computers he has access to. You can find a good list of various monitoring programs here: Monitoring Software Review 2012 | Best Computer Monitoring Software | Monitor Activities on Your Computer - TopTenREVIEWS
Basically you need hard evidence before you do anything else. You need to prevent him from taking things underground. You will also have a much higher chance of blowing up his EA if you can show hard proof of inappropriate conversations to his family, friends, and co-workers, without proof, you could be made to look like a controlling, paranoid, jealous spouse.
A major component and draw of an affair is the secrecy involved in having one. You most definitely can and should ruin the affair, and the best way to do that is to remove the secrecy by exposing it. Once you have proof, and know more about the OW, you will have little difficulty in making things uncomfortable for them both. Best of luck to you, and please keep us posted.