Quitting a postdoc: how to justify myself with my PI?
If he is a reasonable person then be honest. Something like:
I was happy and enthusiastic to come and work with you on the project, but it changed before I started and I don't see any advancement for me in the current situation. I've found another position that I think will be more productive and need to start it soon. I'm sorry that it didn't work out.
Hopefully all of that is true.