Tell him that if " a half truth and protecting her "is more important than the relationship he has with you , it's clear where his loyalties lie.
He has hurt you, not been completely honest and you'd like him to leave.
Remember he is your childs father and so will need to pay child maintenance until you child leaves college.
Tell him you are sad that the relationship is over, but the trust is gone.
Ask him for his keys.
Ring your landlord/utilities and remove his name.
If you are on low income and need support register for Universal Credit.
Single is less lonely. Trust me.
Dont you bother doing the detective work. Leave the past where it belongs and use your energies moving forwards.
You will never know if he did or didnt. You dont need to know who. If there was nothing in it he would have told you at the time " wTF..just got this from Sarah. Told her I'm with you, and I've blocked her"
Men are stupid and have egos. But if he's not doing enough to reassure you after dropping the bomb, he's not for you.
He may well be a better father than partner. Ensure your child sees him and doesnt hear negativity about him. From someone who works with children, he's 4. He will still love the both of you and love staying with daddy. His daddy still loves him. That's all he needs to know.