How do i deal with my PitBulls aggression to other people and dogs?

i have a male PitBull that is a year old and will be 2 in September. He seems to be having aggression problems with people and other dogs especially males but that is usual for male dogs to do. He tried to bite an older man and only caught his Jeans but his collar caught his leg and tore the skin (he has a spikey training volor that kinda pokes his neck when he pulls) but he told the officer that that was a bite mark. He was put under a 10 day quarintine, got his rabies shots (we would ahve gotten them sooner but didnt have the money for it and are still making payments on it) and is also registered with the police department. Ever since that day my dog thinks its ok to try and bite other people (i have no idea what made him want to bite that man the only way he would go after someone is if they went after me) and trys to go after every dog he sees although he is just fine with puppies and of course females. If i cant find a way to deal with his behavior problem soon i may be forced to get rid of him and my dog is my best friend and i dont want to lose him. :'( i have tried emailing dog trainers but sadly they seem to be a bit worried about money which we just dont have at the time or they dont email me back so im not sure what i can do for my buddy.