I agree with everyone telling you to basically keep shtum here.
Talk to your mum, and talk to a solicitor.
You can write a letter to your mum and in it tell her you feel so rotten and also ask her for whatever kind of support you need from her. Tell her that you are not sure at all how this will end up and that you will need her unconditional and unjudgmental support no matter what you decide.
Take the time to let your discovery sink in. You have an ideal setup with a newborn baby - he won't suspect you are devastated by what you saw and will put any upset or change in you down to having a newborn, sleepless nights, stress juggling the older child and the baby.
See a solicitor. Bring your mum along. Get someone to take care of the baby and little one. Before you talk look up all the details you can find wrt your financial situation - bank account/s, mortgage, lease, car payments, student loan payments, cc bills and accounts.
When the time comes, ask him if he's still shagging the same girlfriend he shagged right before your wedding.
Don't let on you have any sort of proof. Listen to him protesting innocence, getting angry, accusing you of being crazy, and all the other ways he will try to convince you that the problem is all inside your head.