My dogs love of others is dangerous!

I have a 16-month-old male Cavalier King Charles Spaniel. He is a very confident, sociable dog with no signs of fear or aggression. He loves meeting other dogs but the problem is he seems to like it so much that if he sees a dog in the distance that he can not reach or if a dog walks past him without stopping for a sniff, he starts barking excitably and trying to pull towards them and will not stop until they are out of sight. It is a little embarrassing as he will not listen when he is in this state and I would like to know how to stop it from happening or how to stop/control if it does happen. It also makes it impossible to let him off the lead at this stage as he would just bolt off after any dog he sees, with no concern for traffic or any danger. He was castrated a few months ago in the hope that it may help, but although he has calmed down slightly it has not stopped it.
Rebecca Otter