I think he is trying to rule the roost. He needs to know you are in charge but not in a mean way. When you get a grey you should not have spent any more time on him than what you planned to spend for the rest of his life. Lavishing all that attention on him at first is what made him think it was going to be like that from now on and it ain't.
I must say that I guess I have had it easy as my grey, who turned 1 year old last month, has never bitten me. She has beaked me and pushed me away with her beak but never broken the skin. When I tell her to step up she does because she knows I mean it. I don't let her get away, I follow thru.
Maybe the kids shouldn't try to pick him up at this point and just you and your fiance do that. Gently but firm have him step up and if he tries to bite don't pull back or flinch. Show him it will not make you go away and he will eventually give it up, I hope. If he does bite tell him no and turn your back to him and walk away. Go back after a bit and try again.
Maybe I am not the person who should be giving you advice as I have not the experience with biting greys. Mine has been so sweet so far but that could change when she goes thru the terrible twos.:dry: