Lab running back to other dogs

Question
Hi,
We have a problem with our 5 month old black Lab. When we take him for walks we always let him off the lead to explore for himself (whilst keeping a close eye on him!) but as soon as he spots another dog, he will completely ignore us and races after them. This is fine up to a point. I think it is good to let him get used to other dogs but the problem arises when we carry on walking. After we have walked a little way further on, he suddenly races back to the dog making it increasingly embarassing for us and the other owners! It now means that we have to put him straight on the lead as soon as we see other dogs in the distance! If not then we think that he'll completely ignore us and just go off with the other dogs!
If you can advise us on how to get him to continue walking with us then we would be very grateful as he really is a beautiful well-loved dog which only has this one problem!
Thanks,
John and Jen

Answer
Got a very good, new, puppy raising manual.  One statement it makes is never give a command you can't enforce.  You can't force him to stay with you off lead around other dogs.  The solution is to not leave him off lead around other dogs.  I frequently take my one dog for off leash walks, but fortunately I have a large area near by largely to ourselves.  

Having him run up to strange dogs like that could be a problem if the other dog is aggressive.  I wish I had a better answer.