First of all, no question or comment is silly, so don't hesitate to post anything that is of concern.
Antidepressants cause mouth dryness.
This is one of the side effects of these medications. The body needs this amount of liquid for all organs to function normally, including heart, lungs, bowels, bladder, etc., and he will be much more comfortable if he keeps himself hydrated. The dry mouth side effect will be minimized with appropriate fluid intake. A partially collapsed lung or some lung consolidation is a serious problem. Again fluids will be helpful in keeping lung surfaces moist and prevent problems from developing such as pneumonia.