Thanks in advance to anyone kind enough to assess my situation.

My little boy Valentine is quite possibly the sweetest glider I have ever handled... to a point. There are times when he’s comfortable enough to lie in my hands with his belly exposed, happily snacking away on a treat without a care in the world; however, this is not always the case.
He’s always been grabby, which I know from personal experience bonding with him, grabbing leads to biting, so I do my best to avoid drawing too much attention to my fingers and keeping his focus elsewhere, and more often than not, things go very smoothly. Unfortunately, there are also times when he will grab ahold of me before I even know whats coming and chomp down, immediately breaking skin. No matter how much I “tsst,” he will not let go. There is no crabbing or anything, just glider teeth in my finger. I would be more patient if it wasn’t so emotionally draining, especially after just losing my little girl (I’ve already begun writing an email to a trusted breeder so he won’t be alone. The biting was commonplace even with her and luckily her passing hasn’t been too hard on him, but I digress).
If anyone has any sort of explanation for why he still does this, any insight would be most appreciated. I’m ashamed to admit I’m reaching my wits end.
My sister and I have suspected there may be something neurological, given that some of his physical features are unusual, his face being a bit smooshed and his tail somewhat shorter than the average glider, but there is no way to be certain of this. He’s very anxious, yet his curiosity often overrides his nervousness it would seem.
Please, I know I’m rambling, but I love him so much and it would mean everything if I could get some advice, and again, thank you so much.

I'll be 100% honest upfront & warn you I do not have a solution for you that is a quick fix, but I will offer some tips & insights which may or may not help. Before we go too deep into your options, have you considered what your boy is smelling that makes him want to 'nom' on you? Do you use lotions or scented soaps? Do you want your hands after handling treats or food?

Outside of that, he may still be trying to figure you out or just, well, stubborn or not very bright. My Arthur likes to grab me digits & if I don't break his grasp, he will bite me. It helps I have tough skin, so he rarely draws blood, but it is not pleasant. Specifically, with him, I will twist my finger slightly if his face comes near the digit he has claimed, & if that doesn't work I'll blow on his face as he goes in for a taste. That discourages him. That is above & beyond me telling him 'no' & 'tssk,' or an outright 'hiss.'

You can also turn the 'hand-holding' into a game. It worked with one of my males (Merlin), but not the other. When he grabs you, more your finger like you're dancing with him, or bring your other hand in the pet them... or both. The idea is distraction & making it a fun or challenging interaction. Both my girls will grab on to my fingers then look at me, which I take as 'what will you do tonight?'

It takes a time, but learn to recognize the signs & body language that says he is about to chomp down. Break the connection or find what will discourage him, it'll (mostly) break the habit. As I said, with my Alpha brat, it hasn't disappeared completely but it is manageable. Also, I know it is hard, but you cannot take it personally/emotionally. That will only make it harder to adapt or adjust your tactics.

I agree with Hutch as I have one that still bites but she does have body language that happens just seconds before. I can't describe it, it's just something we know. I respond by putting her back in her cage. And then she goes through this crabbing thing when you get near her for a few hours. Her bites are like she's fighting for her life... Yes we know she is mentally not right. Since we brought her home. Nothing stops her bad mood except putting her up. Her name is Dot BTW.

One of our other members had a biter too. She has often responded to this issue by having a small toy at the ready. Gliders become fixated sometimes on what they're going to do. It's hard to break their habit. If you can tell he's about to bite, have a small toy (army man, or something they can grab) or a piece of fleece ready. When they get ready to bite, (lower jaw doesn't move, but they lift their head to expose lower jaw) shove the item in the way of your finger. It's not easy and even if they latch on to you, use the item to push them off carefully.

One of my girls, Arwen, still has a biting problem when it comes to crawling on my clothing. Not sure why. I think she is trying to groom but she goes for seams.

One of the boys, Elric, likes to bite fingers and pretty much only fingers. We are working on that and he is getting better. All of them occasionally go for a nipple when crawling under my shirt (I think it is a grooming thing there too) but I can usually tell when that is about to happen now and distract.

All of them used to go for armpits especially Eilonwy until I find deodorant they do not find tasty though I think one scent they thought was pure evil because they went into attack mode.