Hi, welcome to the forum.
Sorry to hear of your Grey's biting issue. More info would be helpful (home environment, how long you've had your Grey, if you have a partner around/does your Grey prefer your partner, sufficient diet/sleep, does your Grey have enough toys, etc.). As cml suggests, could you perhaps post more info in the General Parrot Forum?
Sometimes Greys get in moods when they like to be left alone and not handled too much; does your Grey give you warning signs when he/she doesn't want to be handled? My own CAG has her moods where she loves to snuggle and then she has her moods when she wants me to back off. I think respecting the parrot's boundaries and not push the parrot really helps. If your Grey doesn't want to step-up, for instance, back off and try again later.
However, considering the age, though, your Grey could very well be going through sexual maturity. African Grey parrots go through sexual maturity between the ages of 3 to 6 years. You've basically got a teenager on your hands!

In any event, I think it would be wise to talk with an avian vet. Patience and trainings is key to help this phase pass if your bird is indeed going through "puberty."