Like Dan said, they choose their favourites and it sounds like this is what he has done already!
I am the main carer for my birds and the only person who can handle them, this was through the birds choice not mine. Each baby I have brought home was well socialized and would go to all the members of my family then a couple of months down the line it all changed and would only come to me.
My middle grey will come and sit on my husbands knee and all three will accept treats from the rest of the family. My husband feeds the birds on a morning just to make sure there would be no problems feeding if I am not here for some reason, he also covers them at night.
get other members of the family to help you do this.
On the question about the vacation, will you have someone who will be staying in the home with the birds who they will know? You will need to plan ahead for this and make sure the birds are used to the sitter, I would start to plan this now. They are very sensitive and most greys are very wary around strangers.
Caroline.