Ok, I don't have a pit...I have a Bull Terrier, but the questions you're asking are basic dog questions. So here we go.
1) No it will not effect his muscle growth. I would not advise getting it done before he reaches maturity though. There are some that say doing it too young (before a year) can have harmful effects on behavior.
2)You can get him a harness now, but keep in mind that he's going to grow out of it and you're gonna have to get new ones. If you do decide to get one now then don't have him pulling a lot of weight. I started Sergeant Major (my Bull Terrier) with a gallon of water when he was about 10 months old. It's light enough to not do any damage, but enough to get him used to it. Properly, you shouldn't start adding significant weight until about 14 months.
3) As a puppy and teenager he isn't going to need supplements. You should however look into raw feeding and the BARF diet. I feed Sergeant Major only raw meat (chicken drum sticks, thighs, turkey legs, beef ribs, ox-tails, etc). This requires some research, but yes dogs can eat and safely digest UNCOOKED bones. There are certain bones you cannot feed, even raw. These are bones that are large and weight-bearing on larger animals. However, having a pit-bull you'll be able make some exceptions...such as turkey legs. A lot of people who have dogs that don't have strong jaws can't handle turkey legs...Sergeant Major handles them with no problem and I'm sure your APBT will as well. I also feed him cottage cheese, fresh fruits/vegetables sometimes to go along with his raw meat diet. The thing about most commercial dog foods is that they are loaded with grain. Dogs cannot digest grains!!!! So if you have the time, go raw. Plus it looks cool to see a dog ripping apart raw animal parts.
Good luck!
