He is your best friend, but he is also your little boy, your child. Of course you are upset, and experiencing anticipatory grief while also tending to him.
In my younger days I worked as a pediatric play therapist at a large children's' hospital. One of my rotations was spending a year on the oncology wing as well as working with children with cystic fibrosis--becoming very close to them and getting to know their parents very well. I will go so far as to say you are going through something very similar to what many of those parents went through, while supporting their child in all the ways that they could; without, of course, that last added option.
Birds are wonderful, special creatures. Parrots can be extraordinary. You WILL get through this. There are many online pet support sites that help people who are going through what you are experiencing; and also getting through the quandary of considering that final possible decision. 
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