Running after other dogs is self-rewarding. Your dog gets to the other dog and it plays with him. Obviously for your boy, access to other dogs is far more interesting than food or anything you can offer him. Also, no doubt as soon as you get hold of him, you put his lead back on, so in his mind, coming to you = the end of any fun.

You need to convince him that you control all of the resources he wants, including access to other dogs. Start by keeping him on a long line, as every time he runs off to other dogs and gets away with it, you are reinforcing the behaviour, and the fact that the command 'come' or whatever you are using is meaningless. It means 'come when you feel like it'.

So, dog is on the long line. As he is wondering about, sniffing etc, wait for a moment when he is looking at you, (I find when the dog has got to the end of the longline if I stop walking is a good time) and call him to you. (use a new command, he's already learnt to ignore the one you have been using) Be REALLY enthusiatic, and when he gets to you, give him a tasty treat and then carry on walking. Do it often, call him to you every couple of minutes. You are teaching him three things. One, that coming to you means tasty treats, two, that the walk doesn't continue unless he comes to you, and three, that coming means the walk continues. Do it LOTS, every single walk.

Then you need to find a kind person with some other dogs. 'Accidentally' meet them while walking your dog on his line. Get close enough so your dog can see them (their dog is on a lead and they'll stand still when they meet you) but he can't get access to them. Call him to you. He won't want to come at first, but be patient. Then , when he comes, give him a treat and allow him access to the other dogs. Keep him on the longline. (watching for tangling! ) After a while of playing, call him to you again, and get your friend to take their dogs out of his reach. Give him another treat, and allow him access to the other dogs again. You'll need to repeat it quite a time to build a pattern, but the idea is for the dog to think in order to get access to other dogs, he has to come to to you first.

Don't ever let him off the longline when you think he won't come back. Every time he doesn't makes it even harder to train him not to do it than it was before.

You could also teach him an emergency sit or down. Let me know if you want to know how to do this. I won't now, as I feel like I am going on a bit!