Finn is 14 months old border terrier. He has been well socialised and lives with Tangle a bt bitch (spayed). He was castrated at just over a year as he was jumping Tangle. His mother and father are fine with other dogs and his breeder can't believe his behaviour. As soon as he sees another dog he screams and pulls towards it. He then becomes really aggressive. This has only happened since he was castrated before this he ran up towards other dogs and was fine with them. We walk the dogs on the beach where it is safe to let them off. Just after he was castrated he ran up to a J. Russell and attacked it he did come away eventually when called, since then he has attempted to attack a border collie, so we only let him off the lead if there are no other dogs around. He even screams and pulls when he is walked past a house where a dog is sometimes in the garden even when the dog isn't there! I've taken him back to dog training, but he spends 10 minutes screaming at other dogs and we spend most of the class in the naughty corner, he does calm down eventually but it hasn't affected his behaviour afterwards. What can I do?