What to do with a likely collaborator who is not answering mails?
Presumably your prospective collaborator expects something more tangible than email discussions to come forward soon. So far, what you describe sounds fairly typical - material and ideas are exchanged, and papers are read (or not). However, at some point this needs to move into a more concrete direction than "discussions". That is, you are now in the phase where the pedal needs to hit the metal - somebody, presumably you, now needs to come forward with a concrete action plan or at least steps towards such an action plan.
Otherwise, if the discussions still remain on an abstract level even after multiple months and many emails being exchanged, I could understand that your collaborator fears that they may be wasting their time in this.
So the next step if you want to go forward with this would be to set up concrete next steps: arrange a planning call or even a visit, and then come up with concrete research methods and a methodology to address them. Particularly, be explicit about what you expect from the collaborator - the fact that they were not able to read your papers until now suggests they will be unable to contribute a lot of time, and it is better to clarify how they can contribute early rather than mid-way through the project.
I feel it is important to understand what the power and group dynamics are in such a collaboration. Why is everybody in the project? Who expects to get what out of it? Who is pushing / leading the project? Who has how much time to contribute, realistically?
These questions are all fairly different depending on whether you and your collaborator are students or senior academics. That is, if you as a student cold-mailed a senior professor the collaboration will ultimately have different dynamics than if both of you are students, or both of you are similarly-senior academics.
Meaningful collaboration requires a serious investment of effort. In interactions with a colleague, there is a massive difference between:
"Your work is interesting enough to spend 15 minutes skimming materials and providing feedback, and another 15 minutes looking through your eventual publication for useful nuggets."
versus
"Your work is critical enough to the progress of my work to spend 100+ hours working together on a collaboration leading to a new publication."
From what you have reported so far, it sounds like this potential collaborator is more at the first level and not the second. Remember that most people doing significant research are approached quite frequently by potential collaborators, and simply cannot work with most of the people who might be interested to work with them (or who they might be interested to work with!).
I think that you need to stop and ask yourself not, "What would I gain from working with this person?" but "What would this person gain from working with me?" Moreover, are you sure they wouldn't get nearly as much value just from reading and incorporating results from a publication you make on your own? If there isn't a clear and valuable answer to these questions, it might be that there won't be a collaboration at this time. You could even ask that question (politely) directly and with a concrete action attached, something like:
"Based on our discussion so far, do you think it makes sense for us to try to work together on a project at this time? If so, can we set up a time soon to talk about specific plans?"
This is a hard question to ask, because you might get a "no," but that "no" will be valuable because it will let you move on. Worse, you might also get a "yes" from somebody who has a hard time saying no, but then they don't actually invest in the collaboration, and you also need to be able to recognize that and move on.
Finally, it's important to realize that even if you don't get the collaboration that you hope for, lower levels of interest and interaction can also be quite valuable for your career: even if this person isn't collaborating with you, they may well be genuinely interested and read your work, cite you, pass your work to others, become a good contact in your professional network, etc.
The way I know these folks, he is a writer type, not a reader type. In other words, nobody is going to read your junk unless there is a damn good reason to do so. Find this damn good reason and communicate it or you won't get anyone reading your papers. After he reads your work, take it from there.
Before he says he has read your papers, act as if you don't have any collaborator.
The step after his confirmation - if it ever occurs - could be setting up a plan including the responsibilities of you two and the deadlines.
Now, if you are a PhD student, and the other part is a senior professor, don't count on him reading anything: he is more on the level of managing research and less on the level of doing research. (Exceptions exist, and I know some, but they are not prevalent.) Instead, say that you understand if he has no time to read your papers. Then, ask him to suggest a potential collaborator who does have time, perhaps from his environment. A PhD student who is at the start of his career and has not decided about the direction could be a better match. Another choice is someone who badly needs publications.