I had a coworker do it to me yesterday.
“Thanks for taking care of that issue with Jeremy.”
“Sure, no problem at all!”
“It’s just weird because he told me the opposite thing yesterday.”
Working on Jeremy’s issue and forget to respond…
“Are you mad at me?”
Full stop.
I’m sorry to say that you already know what you have to do. There’s no easy way to do it other than to just do it. If it makes the process easier, you don’t have to pose it as “it’s me or them” because you already know what his answer will be. If he was going to change, he’s had plenty of opportunities already.
Don’t wait for him anymore. Live YOUR life.