Ok - he's still a baby... and I'm guessing your only dog so of course other dogs are going to be a novelty.

When training, you need to build it up. At home, in the garden, out and about, out and about away from distractions. All these have to be trained separately and it shouldn't be assumed because a dog understands recall in one situation it will undestand it in all.... it doesn't.

You've been given some good advice. I personally would prefer a long lead over an extending one. Do you know any other dog walkers that would help you while you are training? Set up a situation where he is with other dogs and work on your recall before he has had all the exciting play. Also, make sure you never allow him to be in a position where he will ignore you. If you don't think he will come, go and get him, otherwise all you are teaching him is that it is ok to ignore you.

You are doing fine, but it can come as a surprise to some how much work goes into training a reliable recall against all distractions.