Im new to pits, i got a now4month old male pit named Ivan. in i have quite a few questions about what to do for him.

1)will getn him fixed effect his muscle growth? wouldnt he not produce testosterone anymore and ther for not grow muscle?
2) Iv done a lot of lookn around the last few wks n i keep readn diff ages to start him at to pull a weight harness, iv even talk to a pit breeder and a few diff friends with pits. iv read 12wks-1.5yrs old to begin the weight training, i dont wana start him to early n hurt him in any way.
3)What kinda supplements can i give him n how much? When i say supplements i mean just that, i am against getn my god geard out cause its just not fair to them, it would b like given a kid gear... Im askn u guys cause ur all atleast sum what like minded ppl n i want my dog to b huge but healthy to. any tips will do... im also putn chicken in his food to get him more protien and make him eat more.