Is he neutered? If not it could be, at a year old, the ol' hormones are kicking in - time to visit Sir Jury! In any case, he's at the age where he will begin to challenge his place in "the pack" and you need to be firm with his training, and absolutely consistent. When he starts to bark at another dog, immediately correct him, and have him focus back on you - do "sit" "down" - whatever commands he knows, to distract him from the other dog, and let him calm down. Don't let him get all wound up.