Sounds like its probably normal puppy mouthiness then. Like with children puppies explore the world with their mouths, it's also how they play and a common behaviour when excited or overtired, so it is a completely normal behaviour. Keep interacting with him, try to redirect him to something else if you can and pause or stop the interaction when it gets too much. A "ouch" or any word you want, it just has to be consistent, when he bites will help him work it out
You can also use food to keep the mouth busy while touching him, so he gets into the routine of not biting every time
It's important to remember he is still extremely young, his brain isn't even fully developed yet. Most dogs outgrow the mouthing and he still has plenty of time! It would also be good to remember that 6 weeks is very young for a puppy to leave its family. The generally learn about biting, whats too hard, when you should stop, and other important social cues from their siblings and mother, so if you got him at 6 weeks he would be missing 2 weeks of learning important social behaviours which will add to the difficulty