
Hello :)
I've been in this situation, but with a 7 stone dog it's a bit of a nightmare! I would suggest a behaviourist - let me explain why. My boy did the same as yours, he was absolutely fine and then one day a dog grumbled at him and looking back at it, my boy was shocked - up until then he'd been loved by everyone! After that he started having a go at shows and eventually got quite a name for himself as an aggressive dog. Outside of the show atmosphere he was just the same dog and he still loved seeing the "show bag" come out and got excited but once we were there he changed into a monster!
To cut a long story short, we took him to a well known behaviourist at the other end of the country on a personal recommendation for an unrelated problem we had with our "pack". We had only been there half an hour when she said to us that he wasn't an aggressive dog, he was terrified! It would seem that this dog having a go at him had upset him and he got fearful of other dogs coming too close and the "aggression" was in fact him telling them to go away because he didn't like it!
We took him away from the show ring and last year he only went to one show. He was doing well until a dog turned to face him in the ring and he was off again :( We'll try again as he matures to see if he improves in his self confidence, but for now we love him for who he is, not for what he could win (or lose!)
All I wanted to say is that "aggression" is often misunderstood behaviour :)