Just got my new atiel today, anybody have any tips?

He is believed to be male, he was handfed, was hatched in August and just weaned about two weeks ago. He's very sweet, I was playing with him in the store and when I got home. After a while I put him in his new cage so he could get accustomed to it. After about an hour, I took him out again and preened him for a while, he loved that. :D Now I just covered him up for the night but before I did I talked to him and pet him a little bit, but he hissed at me first but he still let me touch him. He also hissed the whole time I put the cover on his cage. Why do you think that he was hissing? I know what it means when they hiss, it's just that he didn't do that before. This is my first atiel. I've had budgies before and this is a step up for me. Does anybody have any tips on bonding and stuff, or ideas as to why he hissed at those certain times? Thanks.