My FIL (93) is almost blind and hard of hearing he's unhappy and says he just wants to die. What to do?

He has been in AL about 2 years now, since my mother-in-law passed. He is bored and lonely most of the time; he lives near one of his sons who visits at least weekly. He does participate in activities there, but the loss of being able to read and hear and do other things limits him. He does get audible books, and his other sons call and visit when they can, but they live farther away. When they call, he mostly complains about his life--understandable but how do we respond to this? It seems to me that as we age, we keep losing the abilities to do things we once enjoyed, one by one. The way to keep going is to focus on what we can do and find purpose in our lives. But how do you help someone else find a purpose in life? How do we respond to all his negativity?