It sounds like he is starting to mature and changing. You have to step in and stop his behaviour and soon as you see the signs which are:
•Staring directly at the other animal
•Raised hackles
•A stiff tail held up high may be wagging
•Ears pointing forwards
•Straining against the leash to reach the other animal
•Stalking
When you see this behavior correct the dog, try shaking the leash and saying No, get the dog to focus on you, get him to sit and give him a treat.

since he is young you have an easy case
when your on a walk with him and you see him staring down another dog and growl give him a firm but quick tug on the lease once you have done this realise the tension on the leash, if he continues repeat over again until he stops some tips I would suggest is to watch The Dog Whisperer with Ceaser Millan, he deals with extreme cases of aggression and is even an ownwer of a very well behaved pit bull

You need to show him that you are the leader. Keep him on a leash and give a correction (quick jerk of the lead) every time he displays any signs of bad behavior (hair raising, staring, growling, barking). If he doesn’t react quickly to the correction, try putting a pinch collar on him when you are out with unfamiliar people. He needs to understand from you what is appropriate behavior & what is not.