you should still be really nice with him. maybe try to get him to lighten up. maybe the reason he hates his job so much is because people don't make an effort with him, and so he started becoming bitter thus making other people want to talk with him less. lol it's a big cycle. you should just be super nice to him regardless of how he talks back, you never know what effect your kindness will have.
the other day, i went thru a drive-thru and the lady sounded like she really hated being there. i just talked to her with a very "happy" voice, asked how she was and how her night was going, and although she seemed taken off guard, she responded in a very appreciative tone. after she took my order, i said thank you so much. when i drove up to the window, i paid for my order, she gave me my food and i said thanks again (called her by name) then told her to have a great weekend. she had the BIGGEST smile i have seen on a person and she had a look like she couldn't believe what was happening, lol.
it was fun, felt good to turn her night around