Project is going nowhere, can I nix it and try a new approach without my co-author?
Do you have a supervisor who would appreciate being kept in the loop?

I think your best bet is to give this colleague a clear message that the methodology he has proposed is flawed and that the results coming from that methodology are not promising.  Ask him if he can think of any justification for you to spend more time on this project as currently contemplated.

In other words, pull the plug on the project.  But indicate that if he has any other ideas for collaboration about something else, you'd be happy to work with him on a different project.



If you work up your other idea, and publish that work, make sure to mention him in the acknowledgements.