He's still young, he's learning who you are and one way he learns his surroundings is with his mouth, like a true toddler. He could be scared, you could taste good to him (fruit-scented soap or shampoo?), or he could even be grooming you. Instinctually, gliders use their bottom teeth to scrape and remove bark off branches, so the urge to bite and chew to get food is quite ingrained.
I suggest offering him knuckles and a closed fist instead of fingertips, and spend more time with him in his bonding bag, where he can't bite. Put some you-scented fleece scraps in his bondin bag an sleeping pouch, so he learns your scent. Offer treats by hand. All these things will help increase his comfort level and he won't be as scared. Until he is more bonded, it's not really too great to give him too much space, especially if he hides behind your back and bites when you try to remove him. Keep him in a zipper bag, cup him through the bag, and get him comfortable with you before you start giving him free reign.
I also suggest you don't put him away when he bites. If he's biting out of fear, you're training him that biting = being put back into his cage, so he will quickly learn to bite you when he's had enough playtime.
Instead, try this: When you see he is about to bite, or when he bites, make a TSST noise, short and quick. That is gliderspeak for "no!". You could also say "NO" in a sharp voice. If he persists and ignores this, a quick breath of air to his face while you're saying no or tsst might get his attention. Keep on it, and you'll train him out of this behaviour. My gliders, when they groom me a bit too roughly, get the TSST noise and instantly recoil in shock. It's quite effective.
In my experience younger gliders go through a biting/testing stage, even if they're normally sweet. It's sort of their way of testing their limits, seeing what they can get away with. Since he's a single glider, he can't learn how to behave from interacting with a cage mate, and won't learn what is "too rough", because he doesn't have a buddy to keep him in line. It's up to you.
Speaking of buddies... You should also consider adopting a cagemate for him, ideally another young neutered male. He will feel more comfortable with a glider companion, they will teach each other, and he will warm up a lot quicker than he would on his own.
But it's still early. Don't fret, keep working with him, especially with vicarious bonding methods such as the bonding bag and scent blankets. You'll get there :)