dog aggression

hi, my german shepherd has started to become nasty with other dogs. i socialised his well from a pup and he sees dogs everyday on the park. some dogs he is fine with and some he hates, it seems to be the more submissive and friendlier ones that he doesnt like for some reason
i am wary of letting him off the lead now as i dont trust him. any advice anyone x

How old is he now? Do you attend a dog club?
Is he neutered?

Is he nasty to dogs or bitches or both??? Does he start an incident?

he will be 3 years old in july.i dont at the minute no but am planning to do agility or something but am worried he will go for the other dogs. yes he was neutured when he was about 18 months.
doesn't seem to matter if it is dog or bitch. if a dog comes running over to him he will sniff them and then will all of a sudden lunge and go for the top of the neck mainly or some he is not bothered by. he has never actually caused injury to another dog tho

I think the best thing you can do is find a good trainer....someone who can spent time with you on a one to one...hopefully someone who has stooge dogs to help you.

The sooner you can fix the problem the better, before it becomes a learnt behaviour on his part.

But till you find someone, try and keep him away from situations where he will repeat this behaviour.....the more he does it, the more he will do it.

Good luck