People with pit bulls feed a large breed because they are uneducated about the breed. In no way is a pit bull a large breed dog not even at 65-86lbs that some bullies get is considered a large breed. Now if you have a dog who comes from Bully lines that are over 100lbs then by all means feed large breed food. Lower protein is for slower growth and you really only need that is dogs who grow too fast or in large breeds like great Danes or Newfoundlands.
Deciding how much to feed goes by how your puppy looks I stop feeding 3 times a day when they are around 16-20 weeks old but that is me. The worst thing you can do for a young pup is over feed and get them fat at an early age. Your pup will mature at his own rate and no need to try and bulk him up or really worry about growth for now. He sounds like and average size pup for now and even though his parents were larger at about 65lbs most APBT are closer to 50-55lbs or smaller. It is possible you have a bully instead and not an American pit bull terrier or APBT.
I also would stop the wet food, there is not need for it...... ever unless the dog is sick and you need to entice them to eat. Start good eating habits now
The under bite is no big deal I have many dogs including show dogs with under bites. It is genetic but not the end of the world even in show dogs they can still Ch in the UKC or ADBA. It happens and as long as it is not extreme it should not cause your pup to be any different or any problems. If the under bite is small and your pup is this young there is a chance the bite can correct as they get older and get adult teeth at around 4-5 months old. I have seen both under and over bites correct as the pup grows correct and some never correct just depends on the pup.
I really applaud your for doing research for you pup there is so much misinformation out there about pit bulls and them being large breeds or bully vs APBT. We have many great experienced members here willing to help. As you start to research more you will learn that a lot of what you heard from everyday ppl was nothing more than ignorance, while they have good intentions for you they just don't know any better. You are on your way to becoming educated on the pit bull breed.