He is teething at this age. Give a yep and take his muzzle and GENTLE tell him no bite. Be forgiving and consistant and he'll get it. If you are playing rough with him, you are encouraging him. Give him something else to bite on when he does this.
Also, 7 weeks is too young to leave the nest. They learn a lot playing with their littermates. One is that biting hurts.
I have a 15 week old puppy and she's been very bitey as well. It's only when she gets super energetic and demands that we play or she gets over stimulated. Plus her adult teeth are starting to come in. We say "OW", grab her muzzle and tell her no.
Our struggle is that my bf's two little brothers who live upstairs get her riled up very easily so she every once in a while nips them on the head if they're on the ground. She's getting better at not doing it, but still a struggle.
It's going to take a while, but they learn and start understanding a bit more every day. Just have to keep working at it and be persistant about it.
7 weeks is really young to be sold off. In many states it's illegal to sell pups at that age. Did you do your research on a breeder and get written documentation of all the recommended health exams on both sire and dam?
It sounds like he came from a typically bad BYB or worse. Really good breeders don't let them go until 10 or sometimes even 12 weeks. There is LOTS that the mom and littermates teach the pup that you will now have to step up and do. That's a steep learning curve for you as a novice owner. Set a program and stick with it daily.
You can also redirect the pup with softer chew toys that they need at this age.