I'm not going to tell my half-brother who his real father is
Why would you leave out important information like this?
He deserves to know, I guess. I just want to save him further pain.
It sounds like a tough decision. Your brother does deserve to know who his dad is, though, and you shouldn't hide that from him.
I'm only holding the information in because there's no need for my brother to know. He'll only be hurt by finding out and we can continue our relationship as it is.
As much as it might hurt, you need to tell your brother the truth. I know that he would much rather know the truth than live a lie any longer.