How do I stop my dog going crazy when he sees another dog?

I have a 10month old German Shepherd. He goes to puppy cles every week, he sees other dogs on a daily basis both on regular walks and on trips to the park etc. We also have friends who have dogs which he sees on a regular basis but still every time he sees another dog he either automatically begins pulling towards them, thrashing around to get to them or he lies down on the ground until they come close enough for him to pounce. Even if in another dogs company for a long period of time he won't calm down for ages, he follows the dog around constantly. We have tried everything we can think of to stop this behaviour e.g. treats, walking in the other direction, leave command etc but nothing seems to work. Any thoughts/ideas would be much appreciated. Thanks.