How should I handle this chronically negative family member?

I have a family member who is currently driving a truck over the road for employment and… HE HATES IT. There’s honestly no other job option for him at this point because of his history and there’s a lot of drama about why he even has to work this job. (in short, another family member indirectly caused it) Anyway, every time you talk to him on the phone he goes on and on and on and on and on about how much he hates his job/his life sucks/he doesn’t deserve this/etc. I simply can’t avoid talking to this person without starting a family war. In an effort to salvage some sort of positive conversation, I try to be encouraging like: “Just because it currently seems hopeless doesn’t mean that your future is ruined”, “we’re all looking for a new job for you”...and so on. The problem is, any time you try to encourage him – he blocks you with some kind of new gripe. He is an all around impossible person to encourage and his negativity is weighing my entire family down. It’s so bad that other family relationships are starting to suffer and anyone who has to answer his call immediately gets in a bad mood. I’ve even tried being blunt with him and saying that having such a severely negative attitude about his situation and constantly griping to people isn’t doing anything but making everyone dread talking to him. All that did was make him the martyr of the century. I feel really bad that – at no fault of his own – he has to work a job that he hates so much…. but for crying out loud I have no idea what to say to him anymore!

Am I handling this situation in the wrong way? If so, what could I be doing differently? If you don’t have any advice about that – maybe just give me some random conversation ideas! I have to call him in a couple of hours… so suggestions of ANY sort would be appreciated :)