Aggressive Behavior

Our dog, a 1.5 year old German Shorthair Pointer loves to play with other dogs. Normally, he plays fine, but sometimes when the other dog gets tired (ours tends to tire others out pretty quickly!), our dog becomes aggressive and starts nipping at the other dog, to try and get him to play. We then have to pull our dog away and the other dog's owners usually aren't too eager to bring their dog back. He's not aggressive in any other way (growling, snapping, etc.) Any suggestions to get him to play nice all the time?