You can start a dealing/healing process by giving your son a strong emotional vocabulary. Expressions like "I don't deserve anything/I hate my life" are the end result of a deeper process of feelings, which, if you are to address them, must be recognized and dealt with by the child and yourself/other adults he can talk to.
This is one example of countless emotional vocabulary charts or lists available simple by googling:
Some of these emotions may be a bit sophisticated for a 6 year old, but I think they are still useful if explained well.
After being bullied, "I hate my life" may mean your child feels any or all of the following (or more): isolated/alone, afraid, confused, worried, inferior, frustrated, vulnerable, anxious, etc.
Always accept that the child feels what he feels; you can't talk him out of feeling it. Ask questions encouraging the use of more precise feeling words and listen to the answers. Maybe write them down and ask for what someone's face would look like if they were feeling these things (this will help with empathy later.)
Next, ask what emotions might lead to someone calling him a s***head: jealousy, hostility, angry, hateful, insecure, anxious, frustrated, etc. (It might help to use an experience he had when he called someone a hurtful name.) This will help your son to see that it's not necessarily about him, but much more likely about the person doing the unkind name-calling.
Exploring positive feelings your son may experience throughout the day as well will help to lessen black/white thinking (either all bad or all good).
Read about emotional "intelligence"/awareness, building resilience in children, and teach your child to treat people with empathy and kindness, so that he can feel positive emotions about himself. Let him learn how empowering it is to feel nurturing, appreciated, proud, powerful (one has power over their behavior) etc., instead of worthless!
Obviously, in addition to talking to him about it, you should be talking to the school about their policies regarding the behavior of the bullying child.
Resilience Guide for Parents & Teachers