I have just received a decision letter for my submitted manuscript to an Elsevier journal. It was a revise and resubmit. One of the reviewers recommended rejection and his comment was just "the contribution isn't strong enough". I don't really know how to respond to his comment. Should I write something to convince him that my contribution is relevant or only respond the other reviewers comments? What do you recommend me to do?
You really should respond to all reviewers in your response. If the only comment was "the contribution isn't strong enough", then you need to reply showing why it is, in fact, strong enough. You can say something like "We feel that this topic is relevant because x, y, and z, and this manuscript contributes a, b, and c to the field. We have reorganized the introduction to more clearly outline the need for this contribution, and have added a paragraph to the discussion demonstrating the importance of this work."
The second part is important because it shows that you're actually improving the manuscript in response to the reviewer's comments, not just blowing them off.
Suggestion: In the discussion section, find literature that supports why your findings are important. Then in your response to the reviewers, write, "We have edited the discussion section to provide additional explanation of the importance of these findings. We have also added additional literature that highlight the importance of these findings to the field."