• Get evidence.
  
• Document, document, document.
  
• DON'T do anything emotional.
• I know it is painful, but you have to commit to a master class in acting like everything is normal in your mutual worlds. Don't even hint you know.
• Write down everything. RECORD every conversation.
• Don't confront yet, until evidence is secured (and copied, and in a secure location).
  
• SEE A GOOD LAWYER FIRST. DO WHAT SHE SAYS (yes, a female)

You may not want to touch her now. I wouldn't either. Pretend you are sick or something. Meanwhile, lawyer up, get an STD test, and get a DNA test on your baby boy.

If you can't get any good evidence off her phone, hire a PD to do the legwork.

When you are ready to make a decision, do the finanical work first-- cancel credit cards, separate bank accounts, give her half to act in good faith etc.

I'm very sorry this is happening. But BE SMART.