my 2 year old staffy is becoming aggressive with other dogs

my 2 year old staffy cross has started growling and barking at other dogs in our local dog exercise area where as before, he was really passive
it's not every dog, he seems to have only started it within the last few months and now we dont trust him with other dogs
its a shame as i have just got him to chase and fetch a ball, one minute he is playing happly with the dog, the next he has pinned them to the gound, barking and growling at them
some dogs he plays for ages with them, rolling about and he's not bothered

This can happen all of a sudden with dogs. He could have had a couple of scares from other dogs or maybe he has decided he only likes certain dogs. Is it definitely aggression and not rough Staffy play?  

he pins the dogs down, and really growls and barks at them, he is know as a really nice soft dog but i dont want him to get a bad rep because he's a staffy, it sounds really bad when he growls and the other dog owners get funny with us, do i need to get a muzzle for him

Hey I think its pretty much the same as us humans, we don't like everybody and its the same for dogs. My boy is the same, I don't let him off with random dogs now for this exact reason. I don't think you should get a muzzle for him, he's not really done anything wrong but I do think you should keep him on a long lead and be selective over the dogs he's allowed to play with, I would only let him play with dogs you know he gets on with from now on if I were you.

Why is it they change. You think you have this amazingly placid, submissive, friendly dog and then they start to change