Hi Rafferty,
Sorry to hear of your issue - has it come on recently? Is the dog in good health? Have you changed his food at all or is he on medication?
Did he get attacked by a dog in the past or has his home life changed at all? Are you nervous when dogs come close to yours now and does this affect your posture and reactions when they approach? Are you walking in a new park/ field? Are there new noises going on (ie trams/ tractors)
Apologies - it sounds like I'm prying, but each of the above factors could be contributing to the behaviour that he is displaying and it might help you to understand why he is suddenly reacting the way he is if we can pinpoint what has changed in your/ his life recently to attribute his snapping reaction to other dogs when playing.
It is generally believed that snapping (but not actually biting) other dogs is more of a 'pecking order' situation and the dogs are just telling each other off, but if it is out of character the chances are that your dog is under some form of stress when playing to the point that he snaps to get the other hounds to back off a bit until he is ready to play again.
First thing that enters my mind is this - the dogs playing with him, were they un-neutered and approaching him nose to nose? Were they a little too enthusiastic for him during play? Is his hearing/ sight ok - can he hear/ see them approaching?
If he is in full health and nothing has changed with his regular routine, I would probably suggest that you still let him off the lead (dogs sometimes snap more if they are on the lead) and when he has had a very short play - less than a minute to begin with - call him back to you with a toy and treats before he snaps. Praise him and make him feel really good and safe, then let him go and play again for the same short period before calling him back for treats. Dogs snap when they are getting stressed in a situation.
Gradually increase the playing time and keep praising and treating a lot to ensure he knows that playing nicely will be rewarded.
If he snaps - don't shout, as he will pick up on your stress - just walk away and ignore him - keep walking until he follows you then carry on with your walk as normal when he re-joins you. This way he knows that if he snaps, play is finished and he won't get any praise or treats.
Try this for a few weeks and see how it goes. If he is still no better, I would suggest changing his food to ensure that there are no added colourings - this can also make a huge difference to a dogs character and he may be reacting to these (for example, my dog cannot have Bakers as he goes bananas on it!)
All the best and let us know how it goes
x