You need to redo leash training. Teach him "Heel," and if he starts to pull ahead of you, keep the leash so short that he's right beside you. If he still pulls, stop. Wait, and start again, but as soon as he starts to pull, stop, tell him sit. You'll feel silly doing it, but he will get the message that, if he wants to walk on his walk, he needs to not pull.

Why is he doing it now? he's a teenager, testing his limits - and yours. The sooner you correct this problem, the better off you will both be.