Why is my rat biting me, after having him for 6 months?

Have you made any changes in his environment?  Is something scaring him? When you pick him up, make him feel secure by supporting his back legs. Perhaps spending more time with him will help. Are you wearing something that has a different smell? Once you can discover the reason for the biting, you can fix the behavior. If you continue to have problems, you might want to consult an animal behaviorist.