Hi pilou,
Firstly, I am very sorry that your husband is dealing with PC. Weight loss is typical as is depression in some patients. There are medications that your husband can receive for depression, but, sometimes introducing artificial stimulus can have other adverse effects. Something to think about and discuss.
Chemotherapy is given to improve quality of life, but chiefly in the hopes of shrinking tumors and in some instances, eliminating them depending on the location, severity and maturity of the tumor(s). In some cases there can be a complete cure. At the very least it should help in keeping tumors from increasing in size and spreading further. Has there been any bloodwork done to assess the situation of the mets on his liver?
I would strongly suggest arranging for a discussion with your husband's oncologist, to ask these same questions of him or her, and to discuss what medicications your husband might benefit from. The side effects, pros/cons.
Some suggestions to take your husband's mind off of things, might be to get involved in a home project of some sort if he is feeling up to it. Getting out for some fresh air every day can be beneficial. Keeping up with regular routines. Your husband might also benefit from journaling. Writing his thoughts down will help to give a voice, and perhaps provide a release of sorts.
I hope this has been of some help. It is very frustrating I know. I wish you both peace and much strength in the coming days, and all the best for positive results with the chemotherapy.