I know that hand raised grass keets can make great pets but normally when they are parent raised they are best as aviary birds. If yours was parent raised and has never been held it doesn't mean he will never be handable it's just going to take a lot of patience and trust building. You never want to just put two birds in a cage together right off the bat and if you are wanting this bird to be a pet I would definitely not house it with another bird.
Just take your time with him and understand that he is going to be scared at first. You have to show him that he doesn't need to be afraid of you, sit next to his cage and just let him watch you for awhile, once he is calm with you near him start trying to offer him treats through the cage. I'm not going to lie, it may take months of hard work to tame this bird down and you have to be consistent, as tempting as it is, don't grab him out of his cage and make him do things he doesn't want to do, all that is going to do is lessen his trust in you even more.
