(Peter Jansson's answer is interesting, and I personally do like the idea of documenting contributions more explicitly than is currently fashionable. But I think it is fair to say that this more like tomorrow's solution than today's: though people like us would like to see these protocols followed, at the moment those who follow them assiduously seem to be in the minority, so it will probably not be effective in the short term to try to hold a "lagging" collaborator to this standard.)
In my opinion there is relatively little you can do to force your colleague to pull his own weight on your current and past projects, but on the other hand your situation is not so bad.
Here is the problem: In the last two years, I have taken initiatives that have led to a number of publications.
That is certainly not the problem! Rather, that sounds great for you.
My collaboration with X was not so close, since I ended up doing 80% of the work, in addition to having the original ideas, and he/she was only helping by e.g. writing a Section, having general discussions, or giving general comments.
At least X did 20% of the work; that's a lot better than nothing. Seriously, a lot of people on this site are complaining about being pressured or forced to add coauthors who have literally done nothing helpful on the paper, or who have even dragged them down / wasted their time. I think that in most academic fields, 20% contribution is certainly sufficient for coauthorship.
In my domain, the order of the authors in publications matters, so in our "joint" publications I am the first author, but is this enough?
That's really the good news: by consistently appearing as first author, you are getting the lion's share of the credit. If it makes you feel better: I work in a field in which author order is alphabetical, and in my experience it is relatively common for some coauthors to have contributed less than 20%. So cheer up. By the way, your quotation marks around joint look a little uncollegial: your coauthor did some of the work, so it is joint work. This and other clues make me think that you may be overly worried about this and perhaps pushing a little too hard.
What can you do in the situation?
1) If you don't value someone's contribution, don't pressure them to include you on their own projects. Moreover, if you feel like someone is not pulling their own weight on a project, adding a project where you don't pull your own weight may seem just with respect to the two of you, but from the perspective of the larger academic world you're each trying to take credit for more work than you actually did. It would be easy for most of us to multiply our apparent productivity simply by "exchanging more papers" with our collaborators. We must resist that.
It sounds like you are, or perceive yourself to be, a more productive / serious / insightful researcher than Dr. X. If so, the story you want to tell is that Dr. X did some work with you but that his contribution is not a major part of your research program. You accomplish that by doing your own work, not letting your interactions slow you down, and making sure that in your joint work the primacy of your contribution gets documented (as it has been).
2) Make sure that your supervisor knows the situation and can speak and write accurately about your respective contributions.
If you are getting first authorship every time and your supervisor has told Dr. X that he needs to work harder, then you are laying groundwork for him to describe you as being the prime mover in your collaborative work with Dr. X. It would be a good idea to speak to him explicitly about this. Assuming he agrees, he is then ethically obligated not to try to characterize Dr. X's contribution as equal to your own. If you are in doubt that he might not see it that way, I would suggest erring on the side of showing (not telling) your supervisor how capable and productive you are. You do this not by complaining about Dr. X -- just get the facts across; don't rub it in or be bitter about it -- but by going on to do more and better work.
3) If you don't want to collaborate with Dr. X in the future unless he does at least X% of the work -- you can choose a specific value of X, at least up to 50 -- hash that out with him now, before you work on any new projects. You are more than within your right to do so. Indeed, as Peter Jansson indicates, that is a best practice.
