This is something often put down to 'bolshiness' in 'teenage years' and infact it is a warning shot across your bows. Any of this lunging and 'come on then' football yob type behaviour should not be put down to something that will pass in time. I know you are having him castrated but that probably won't change this as previously mentioned.
Basically he is a coward. He is doing this when the other dog cannot retaliate, he is not stupid They are on a lead or behind kennel doors. It CAN sometimes start out of fear aggression after a couple of attacks on him (depending on his genetic temeperament and confidence level will depend HOW many attacks or negative experiences need to take place before he starts to change and have this 'I will punch you before you punch me attitude).
What is of concern is that this sort of behaviour tends to escalate. What happens is it is a negative cycle because at some point he either does it to 'the wrong dog' and is seriously attacked or you get so tense at every dog that approaches, because you know what will or probably will follow from him, you completely renforce his idea that he must take charge of the situation and approaching dogs = need to be all macho and hard to prevent attack.
I would not underestimate that this will escalate. You will probably start to manage it better because you expect it but that doesn't help improve it at all. I really would just invest in a one to one session with a good behaviourist or experienced trainer whom you have explained what is starting to occur and she or he feels they can help you positively.
IF this dog has never had any real negative experiences, but has started behaving in this manner do not fool yourself into thinking 'he is just a dominant dog'. Infact he couldn't be further from it as a dominant dog would avoid a fight at all costs. He is a thug right now. Is starting to learn to be a bully.
Now you can fight fire with fire and take this situation properly and very formly immediately in hand. But I would very much strongly advise you to seek professional help with doing this as this is the shot over the bow you mustn't ignore, especially as it is become more common. Dogs are not protecting US when they are on the lead reacting this way, they are protecting THEM. he lacks socialisation with other canines it seems, even if you HAVE socialised him, more work needs doing under a controlled and relaxed environment. So get out with friends with sociable, easy going dogs as much as you can. let THEM off but keep him on just to see he is going to be manageable, then let him off somewhere nice and open with them to interact and have a good long walk without nagging at him. Do this as often as you can.
If he changes and does attack when offlead, or act as if he will, DEFINATELY seek help immediately and muzzle him until you can. Reason being vunerable dogs will be his 'prey'. Young dogs, dogs on leads, puppies and so he will quickly find himself in deep trouble if he goes for such dogs unmuzzled. So cover yourself and him.
This all sounds all terribly dramatic. Its only him being a bit of a bully... but this will escalate so use this time to work on this. If you don't, time will not heal this problem and walks will become a total headache.
Di
