Introducing puppies to strange dogs

I’ve had my male chihuahua puppy for 3 months, he lives with 3 other dogs and gets on great with all of them but when he is out for a walk and a strange dog runs up to him he gets scared and runs as far as he can on his lead. If the other dog doesn’t leave him alone he snaps at the dog.
I don’t like this behavior but do not know how to change it because he is good with the dogs he knows.
Can someone please give me some advice.
Thanks
Krystle

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