That doesn't add up.... Do you know for sure that he is with someone else besides the text? If he is then clearly this voice that he hears is the wrong one and hopefully he realizes it in the future and repents. I say get confirmation and if it is true then understand that he still needs help and that God loves you. It's hard to see the ones closest to us make the wrong choices and we wish we could take on their pains for them so that they can open their eyes, however, God does not want us to damage ourselves in the process of helping others. The first step is to forgive him and let him know that you forgive him and let him know that if he is hurt that you will try to help him if you can. Now, you have to read up on forgiveness and refresh your thoughts to seeing him as still a child of God who makes mistakes. Relationshipwise, he has ended that by letting go. The worst is really not the worst because some poeple have children involved that makes the healing process harder.
As I said though,make sure to get confirmation!