pulling and barking on lead

Hi my GSD is 10mths old and been well socialised from a young age and passed his basic training class. He was neutered at 6mths. He has become so bad on the lead around other dogs I had to take him out of dog training class as he was barking at all the dogs and seemed a bit aggressive as his hair was standing. He is so good in the house and of lead but Im wary now when I see other dogs. I have tried tips from me or the dog and dog whisper but he egnores me when he sees another dog while on his lead or at training class. Does anyone have any ideas as I cant understand why he has developed this behaviour.

There could be many reasons that are difficult to answer without seeing and knowing the dog

He could be excited and want to play with the other dogs
He could be scared and want to chase them away
He might be in pain or have some other medical condition

I would first speak to your vet and the trainer in the class, they may help you diagnose what type of behaviour it is, and I would consider a good behaviourist to help you with this problem as it could get worse - if he is insured then you might get behaviuor help free

You say he passed he basic class - so he did once listen to you in class, is he able to focus and work with you if you are far away in the corner of the class??