Hi, and welcome :-)
He is still a baby , but thinks that he is an adult. He has reached what we call the Kevin phase (Think Harry Enfield ;-) ) Teenagers can be very difficult, and dogs are no different. You need to go back to basics with him and not let him off lead, if he is going to meet other dogs, for a while. Training classes might help you become aware of where there is a problem, if there is one. Take treats out with you and try to distract him before he spots another dog and reacts.
He should grow out of it, but it will take time and effort. Good luck with him :-p