@Kiwi and Mitch
I just read this 5 page thread, and I must say tell you a few things..
First, I was SOOOO glad to read that you chose to get "the" bird!!!!
I giggled to myself in excitement for you and the bird when read that part.
Second, I admire all the research and advice you seek, it will be forever beneficial to all of you.
Third, I would like to tell you a little story about my CAG, without making this a novel, hopefully.
Baby was literally hatched from an egg by her previous owner, who was an elderly woman who used to breed parrots. It was just her and her 4 birds until she was too sickly to care for even herself anymore. We adopted Baby into our chaotic house with kids and dogs and other birds when She was 4 years old. She adapted quite well. She fell in love with my husband, and wasn't sure about me. After about 5 months she decided that she hated my husband (I said he was smothering her, he disagreed...). She peaked an interest in my 4 year old. She begged for his attention, but he was leery of the big bird. In time she would climb down her cage to see him and give him kisses (with adult supervision of course, Levi asks permission first). Baby now loves Levi and I. Her and I have formed a trust that I wasn't sure we'd ever have, and the same goes with her and Levi. She does not like it when anyone but me messes with her toys or moving her cage. She's gaining more trust with my oldest son as time goes on as well. Everyone in my house is worthy of receiving her kisses apparently. We've had her just over a year now. My point is, only time will tell. Patience is key. Perhaps if your fiance spends some extra time just talking to him and offering treats, a better trust will form there. That's how we did it here, but every bird is different. So I can only tell you my experiences and wish you luck with yours.
I wish you all the best!! You're already doing wonderfully!
PS I failed at avoiding a novel, I'm sorry.