You let him grow naturally (being sure to feed a high quality diet that does not promote accelerated growth) to avoid aggravating (if things are done improperly your dog can have some major problems where there very well might not have been otherwise) bone and joint problems (like hip and elbow dysplasia) and then if you prefer you can start conditioning him properly in a way that's safe for him. There is usually no need for this once they've reached their full growth between 2 and 3 years old and they are quite strong enough without help :-). You need to be sure they are trained properly and that you are able to control them. Here's some links to some information and maybe a club or two you can get involved in to make sure you are doing everything in the proper manner.
www.wpbtca.com
www.pitbulllovers.com
www.pitbulllovers.com
www.pitbulllovers.com
www.pbrc.net
www.dogfoodanalysis.com
www.bornfreeusa.org
www.thedogsbone.com
www.thepetfoodlist.com
www.wpbtca.com
www.pitbulllovers.com
www.pitbulllovers.com
www.pitbulllovers.com
www.pbrc.net
www.dogfoodanalysis.com
www.bornfreeusa.org
www.thedogsbone.com
www.thepetfoodlist.com