"How To Stop Him Biting!" is a question that is often asked about GCCs, as they are, unfortunately, quite well known to be a rather nippy species of parrot. There may be several factors contributing to his biting, so you will need to observe when and where it is occurring. A very wise person (not me!) once said that the best way to not get bitten is to avoid being bitten! Which sounds quite simplistic, but a big part of avoiding a bird biting you is to avoid the things that may be triggering the biting in the first place. GCCs are also quite notorious for a little thing called "cage aggression", where they see their cage as their territory and will often defend it most vigorously! If Koko is biting you when you are handling him in or around his cage, then he is telling you that you are invading his space. Try to ensure that he is out and away from his cage of his cage when handling him, changing food and water, cleaning the cage etc.
Does Koko have access to a hidey/cuddle hut or anything that even remotely resembles a nesting site either inside or outside his cage? If so, it needs to be removed as they can be a real trigger for hormonally-charged biting and aggression. My little lorikeet Lilly is HUGELY hyper-aggressive during her long,
long, breeding season and matters are made even worse if she ever has access to anything secluded or nesty, so you definitely need to ensure that he does not have access to anything that can be perceived as a nesting site. At around 2 years of age Koko is likely heading towards sexual maturity and a lot of biting and aggressive behaviour is very closely linked to this period in their lives too.
Many people use a method called "shunning" in order to cut down on biting. This works best if the bird actually
wants to have a relationship with it's custodian - otherwise you may just be giving him exactly what he wants, so you will need to assess for yourself if this method will work for you. It involves, every time he bites you, you gently pop him down somewhere neutral and safe, like on the back of a chair or on a table. Don't take him back to his cage as, again, this might be exactly what he's trying to achieve. It's ok if he makes his own way back there, but you should not be the one to take him there. Once you've popped him down, turn your back on him for a good minute or two and make absolutely no eye contact with him. Birds in a wild flock would be shunned or even ostracised by their flock mates and told in no uncertain terms that this type of behaviour is not acceptable, and no small prey species like a GCC wants that! In order to work best, this method needs to be repeated consistently by you and any other members of your household. GCCs are very smart little birds and it won't take Koko long to realise that fun times with you stop when the biting starts! It by no means that you will never be bitten again, but it should help to cut down the instances of severe biting a lot, and hopefully save your fingers in the process.
I wish you both the very best of luck!