Bichon agressive towards other dogs

Question
We have a 3 year old bichon frise. He is very sweet to all humans, and gets along well with other dogs in general. Whenever we take him for a walk to the park across from our street, he is excited and happy. When he sees a dog larger than him, he usually hides behind us or wants to be held. When that dog gets near him, he snarls and tries to bite, etc. We figured this was the result of being scared, as he is fine with smaller dogs. When we go to pet another dog, no matter the size, he comes over and bites, snarls, etc at the dog. He does not do this to our other dog, a wheaten terrier. We are afraid he will harm another dog. Why does he do this, and how can we stop it?

Answer
This is an issue of retraining, keeping on a leash close to your side.
Encourage healthy sniffing, most of what you are saying is being orchestrated by your nerves, its quite normal for dogs to sniffad make a noise upon greeting, what usually happens is that owners get anxious and this is sent down the leasto the dog, so the dog gets anxious and hey presto, dog snarl 101.
Just start a good socialising training group which will help him alot.