When a contributor stops contributing In our research I had many meetings and discussions regarding our work (i.e., solutions, implementation, related work, etc) with three other researchers. When I wrote the papers, they were also responsible for reviewing the manuscripts and making decisions together about anything related to them (publishing, editing, assessing comments from journal reviewers etc).

These contributors were my coauthors in every paper I wrote for this specific research. They are all researchers, PhDs and teachers in different institutions.

There are still two papers left: one in the process of being published and other brand new. However, one contributor simply stopped contributing. He didn't say that explicitly, but every time I write a new paper version and our reviewing process starts, he either does not respond to e-mails or takes too long to answer or finish his duties. Usually, the reviewing is only partial (e.g. "I couldn't read all the paper, so here is my comments regarding the first two sections"). 

My question is, even though this person contributed to our overall research and for the papers in the past, should I put his name in the next paper, assuming he is not going to contribute anymore (while the others will)?

How would you deal with this situation?
If you plan to change anything in your authorship agreement, you should bring this issue up as soon as possible.

You could politely ask if the person is still interested in working on the paper,  e. g.  ask "I've noticed that your responses for this paper come slowly. Probably you are busy with other things. To complete the paper I would need... from your side. I could also try to finish the paper without you, but wonder if you still want to be a coauthor." (Just a first shot for a formulation...)

You don't know the motivation on the other side, so it may well be that the coauthor is really busy, but feels obliged to continue to work on the paper and they would be happy to say "I'm out." I have they can't work on the paper but insists to be coauthor, that is another topic for which your field plays a large role.