As hailey said dragons have incredible personalities and can just be brats some times.
With your little one being so new he is probably still sizing you up. The open mouth and backing away is a sign that he sees you as a threat.
Be sure not to approach him from behind or the side, this is predatory behaviour. When you approach him lay your hand flat in his tank infront of him so he can see you arent there to grab at him. If you need to pick him up for baths etc slide your hand under his belly from the front. He's likely to stay the calmest if you use this approach.
Also whilst you are getting used to eachother it can work wonders to put one of your old tshirts inside his viv (do wear it for an hour or so before hand to give it your aroma). This way he will get used to your smell in a non threatening manner.
Remember even if he starts grumpy behaviour with you it can always be rectified.
I rescued an adult who was practically feral, she bit me, hissed at me and scratched me. But with a month or so of hard work she turned into the most affectionate cuddlebug around.
So there is always hope ^_^
Congratulations on your new baby.
Holly