I started a research project in pure math with a coauthor while we were overlapped during my first postdoc and his graduate program. We then moved on to other institutions but have ended up at the same institution again, as second postdocs for both of us.
In the first three or so months of collaboration, we proved several results but not enough for a full paper. After the initial bout of results, which I feel like were primarily my input, he seemed to withdraw from the collaboration, e.g. repeatedly rescheduling meetings or only engaging in the material during the meeting instead of thinking about the problems leading up to the meeting. I set the project aside, but since we're at the same institution again, I figured we could pick it back up. However, throughout the last academic year, I have asked him several times about meeting, and he keeps putting me off, but assuring me we'd find time to work on it again.
I want to finish the project, but because of his repeated delaying, we have yet to meet seriously about it even once since the initial discussions, so I feel like it would be best if I continued it on my own. I also have a sensitive career reason to pursue the topic solo.
Does anyone have any advice for politely broaching the subject of continuing the project on my own? I plan to invite him to look over the file and remove anything that he feels is mainly his contribution, and assure him that I am interested in collaborating with him in general even if I want to pursue this one alone, but I wanted to seek advice on any other way to approach the discussion.