You don't have to ask him to find out who she is. You can check his phone records, the number will probably be obvious. The you can use reverse lookup or have a friend call the number on a pretense.
Reconciliation is a thought for another day. I know, you have three kids and a lot invested so of course you hope for a reconciliation. But take good stock of the relationship and get real clear on what you would need from him before taking him back. I suspect once you have some time away from him you'll start to see the unhealthy things in your relationship more clearly.
