I'm not going to tell a friend that I'm dating our mutual friend.
Is there a reason to hide this?
We're not supposed to be dating.
I understand how you feel, but it's not a good idea to keep secrets like this from your friend. If you don't tell him, he might think that you don't trust him and not want to be your friend any more.
It's a small problem, I don't want to talk about it and to be honest, I don't like him.
Your friend might think you are being deceptive if they find out. You should tell them the truth and be open about it they might come around after a while.