O! M! G! What a fearless, active, playful kitteh you guys have! He is amazing!
I loved the videos! Our first cat, Inky, would fetch like Cooper, and I loved the ninja vid with him and the feet running back/forth.
...and...I think I see what the 'problem' is.
Your vet is right, his being taken from his mother so young is problematical, but the vet is wrong because it doesn't make kittens 'mean', but what it does
mean, is the kitten didn't grow up with a parent and/or siblings to wrestle/play with, to bite and get bitten and learn how to play nicely with others. So...when he came to you, you and your gf became his 'play toys', but since you didn't know how to respond like another cat/kitten, he has not learned to temper his playing.
Someone here suggested when a cat/kitten bites you, push your bitten part towards their mouth. This confuses them, because most 'prey' moves away and inspires more chasing/pouncing. Pushing towards them makes them let go and re-evaluate what is happening. Also, when they bite, make a small 'ouch' cry and stop interacting with them. Move away, push the cat off your lap and leave him alone with no attention for a few minutes. He will eventually learn that the unwanted behavior doesn't give him positive results.
Another thing from the videos...he is just very, very
active! It may take him a while before he 'slows down' enough for cuddling. Right now, it is all about the playtime!
You've got a great cat! I wanna come play with him.