Mini Schnauzer obedience

Question
My 6 month male has come on well with his training to stay, sit and come at home. In the park he is well behaved until another dog is in sight and he will just bolt off after it.  If I spot the dog first I can stop him with constant stay commands, but one millisecond out and he's gone.  He will not stop when called or return when called back.  i always have to catch up and take him away after letting him have a little play.  I do reward with treats if he comes back when I call him, but he will only do this when sniffing alone with no other dogs around.  I want to know a good plan to work on to overcome this major obstacle.  It is making our walks soooooo frustrating.  Is it just his age??

Answer
A lot of it has to do with his age (flight period starts after four months of age), but it will be quite some time before he is reliable off-leash (if ever), so I would **NEVER** allow him off leash away from home at this point in time.

The problem is that every time he runs successfully (AND gets to play with the other dog to boot!) he is being rewarded by his own actions alone, so he will continue to do this behavior until it is extinquished by either negative reinforcement (ie. training collar and a long line) or is forgotten over (a long) time.