that's terrible! Mine never bites at all unless you put your finger in his mouth while he's gaping. GF wont do THAT again lol.
Now to start I don't know a lot about beardies yet but it would seem to me like some sort of trust issue on his part.
Does he show any other signs of aggression when you try to handle him? Like
black bearding or puffing and gaping at you?
Does he try to hide from you or just doesn't seem to like your hand in his "area"?
If it were me I'd try and do two things:
1. get him used to your hand in his area, regardless if he tries to bite you or not.
2. Get your beardie to associate your hand/scent with food/feeding
Give him some superworms to bite instead of a finger, or when you put your hand in his tank have some crickets that you can let go at the same time. That way he wont see you as a threat but as a provider.
Like I said I don't know a lot but it seems like it would be worth a try to see if any of that advice works!
Good luck!