Time to get a long line...
That way he can't run past you, as you can step on the line to stop him.
When he *does* come to a stop in front of you (with your assistance!) reward him by drip feeding several high-value treats - experiment with the treats to find something he does value and use this only for recall at the minute - while doing this hold onto his collar for a while too, so he gets used to you holding his collar without it meaning the end of all the fun. Then send him back to play, explore or what-have-you...
Or is he toy motivated? A game of tuggy can be a fantastic reward for a recall.... and you can use the tug toy to help get his attention - tie it on to a longer peice of rope and run with it dragging on the floor... squeaky toys can also be helpful for getting attention when they are distracted (we ended up with a squeaky toy, threaded onto a ball-on-rope tuggy toy).
Practice recall when he ISN'T playing with other dogs - so the distraction level isn't so high... ideally recall him just before you meet some dogs he is likely to play with, then you can reward him by sending him off to play...
Don't use your recall command (verbal or whistle) unless you can guarantee that he will obey - if that means you only use the command when he is 3 feet away from you and you can see he is coming to you, so be it...
To get him to start coming to you, do the being exciting, high-pitched voice, running away, dragging the toy stuff...
If there is anywhere secure you can practise, try hiding from him.... don't be where he expects you to be - if he is playing, don't stay still, move about so he has keep half an eye on what you are doing - don't be predictable...
