Thanks Tracy. I'm going to go with the medium. Since he got sick last month, his weight has been steadily increasing each time he's had a vet visit, so I'm sure he will only be getting bigger.
Yeah he's light for a DYH but at least he's not over weight which is common with Amazons. He'll probably be at a good weight when you get the harness. Pepsi was at 570 at her first vet she looks like she has lost a bit but she is standing taller and being much more active so hopefully its a good thing!
The harness is great Sisqo will love getting some fresh air and sunshine when it warms up.
Last time I got his wings clipped, the guy told me that he was a little underweight and that I should feed him Taco Bell to fatten him up, because that's what his brother feeds his Mealy Amazon. He was completely serious too. That's the last time I'll go there.