Work on your language; do this with and for yourself so that it is natural when you do it with your MLCer. Validate--Life is Hopeless--without agreeing; reassure him and let him know you realize he feels this way. To disagree is to say the statement is untrue, shows you are not listening to, and thus not taking him seriously. Show complete faith and belief in him in general; he is a good person, good in his profession, good spouse, good parent etc. The next part is subtler.
Words to change
Sounding upbeat and acting if are important, but there is at times there is an obvious undercurrent of inauthenticity when being perky is not the appropriate mood for the context and in such times false perkiness is not how you need to show confidence because it may seem like denial or that you are not paying attention--intentionally by ignoring or unintentionally because you are concerned with your own agenda or issues. Instead be firm in your belief. This is Ericksonian language. Milton Erickson was a Psychiatrist and one of the world's leading experts on hypnosis. Here a few techniques to use.
Some MLCers seem certain regarding some actions--such as leaving, others come and go. But MLCers cycle in their behaviors and moods. Regardless of the mask they show, uncertainty simmers beneath the surface. Your certainty that opposes their idea can plant doubt. I chose to believe that regardless of what I saw, Sweetheart was not happy at the alienator's. Sweetheart showed a lot of evidence toward unhappiness, but this may have been because of my certainty; he fooled others. Even when he seemed or acted happy I reminded myself that it was not real; I believed with complete certainty that it was not real. My certainty caused Sweetheart to doubt himself. I seemed strong, happy, confident--qualities that instilled trust and he thought that I might know something; maybe I was right. I manifested my reality.