How should I manage my relationship with a sneaky academic? I feel let down and (right now) rather upset about how I've been treated by an academic at another institution. Any tips for how I manage my (unavoidable) working relationship with him going forward?

I'm a PhD Student in a small subdiscipline (only 2-4 of us in this country). He is a (moderately) established researcher and for him this is just one string to the fiddle. We've met a couple of times at conferences and organised a symposium together last year. I invited some of my non-academic network who are wonderful collaborators. 

After the symposium we discussed further work together. He asked if I would assist with chairing a (broad discipline) conference, and I was excited about us working together, thinking how great it was to have a good relationship with someone who was more focused on our subdiscipline than my supervisors are, and looking forward to post-PhD collaborations. I felt he was 'taking me under his wing' and that my expertise would be highly useful to him.

Since then, the role we had discussed for conference organisation went to someone else, without him informing me. 

And I've found out he has been putting together a project proposal with my wonderful collaborators, without mentioning it to me. Naively, it never even occurred to me that someone would do this. It seems particularly weird because geographically I'm located right next to these collaborators, and there are plenty of similar organisations in his area that he could have reached out to.

Right now I'm feeling pretty raw about this. (But perhaps this is on me? Was I simply too trusting?) But we are organising another symposium together. Undoubtedly we will cross paths again in the future, and will be reviewing each other's work. 

Any tips on how to manage this relationship going forward? Or other tales of betrayal to help me put this in perspective?
Until you know a bit more, I'd start with a generous interpretation. Academics can be forgetful. They (we) can lose things like contact information. This interpretation may not be warranted, of course, but it is usually worth starting as if it is. It may be that what you were thinking of as fairly firm commitments, he was just musing about. It doesn't reflect well on him, of course, but your best way forward is to assume it is benign (for now).
Send him an email reminding him of past conversations and suggest that you are interested in collaboration. Send whatever support information you think useful. Go visit in person if that is feasible. Remind him of those conversations and your memory of them, but without being accusatory.
You will learn a lot from the response. In particular, you should learn if the generous interpretation is the valid one. I don't know that you have a lot of recourse, however, if he is a bad actor. But if not, you may be able to get the ball rolling on this or a similar project.
If he is a bad actor, you should probably have a conversation with your collaborators about how he treats them and try to figure out if you are treated differently. You and your collaborators can, perhaps, come to some understanding and even create a support group.

On Forgetfulness
One of my mentors, great person, great teacher, used to know when he was &quot;working hard enough&quot; when he would lose his car. You would find him wandering through the university parking lots looking for it when it was time to go home. Not a bad actor, but not always dependable.