Hi and welcome to YT! Dexter is just a cutie pie
Yes, Michelle is absolutely right. It is between 8 weeks and 12 weeks that puppies learn bit inhibition and how hard is too hard from their mom and from playing with their siblings.
I agree with choosing whatever command you are going to use for "no" (said sternly..not yelled or anything). After doing that, put something in his mouth that it is okay for him to bite on (toy, chew, etc) and say "good boy." This will help him learn that biting the right things is okay...because believe me..he will be biting for a while as he goes thru teething.