So, I understand from your post that you didn't try to get him to stop biting when he was younger. How long has it been since you've been working on getting him to stop?
I find that when most people want their puppy to stop biting they try something, and then they try something else, and then they try SOMETHING else, because they get frustrated when they think nothing's working.
The problem with that is, that's just confusing to the dog. You're actually trying to TEACH him something, and puppies don't learn overnight. Some learn faster than others, but it's not really overnight!
So, the important thing is to pick a method and stick with it! Don't give up. Now, with the method from "The Bite Stops Here", you can tweak it a bit to make it work for you, but the general premise works for MOST people. Not all, though.
I know you said you read it, but I'll just highlight a few things:
- the first time he bites/nips: say "ouch!" or yelp: Some pups get more excited by a high pitched ouch or yelp, so you might have to try a lower, more deep sound.
- if he does it again, make the noise again, whatever ended up working (high pitched or low), and then leave the room for 20-30 seconds: This can be another problem people have, if they leave the area for too long, the pup forgets what happened and they'll never make the connection that you left BECAUSE they bit.
- then, basically start over.
But, consistency is the key. You have to do the same exact thing every single time he bites. Everyone in the household has to do the same exact thing. Also, beware of "extinction burst". It goes something like this (from your pup's point of view):
"I USED to be able to bite and nip all I wanted. Now, they're trying to make me stop. Uh, NO! I like biting, I'm just going to keep trying harder and harder and harder, because I USED to get away with it! "
So, your pup may try harder and harder to continue doing something he likes and used to get to do, before the behavior actually does fade and become extinct.
The other thing is, keep in mind that lots of puppies are teething at about 4 months old. So, this is even more difficult of a time to try to get him to stop. Make sure he has lots of appropriate stuff to chew on. You might try wetting a washcloth, tying it in a knot, and freezing it, then giving it to him to chew.