I do agree/feel that this may well have been the problem, thank goodness the vet found it and it was sorted. I do have a belief that many dogs - of course not all of them! - do have health problems that often remain undetected and that in some cases this can cause aggression.
In the wild, an animal daren't show weakness, and especially pack animals such as dogs because they all work together to bring down their prey and although they do someitmes "look after" theweaker ones, it isn't a position a dog would choose to be in. So often it is a similar case in our own homes, with the dogs stoically pretending all is fine and they aren't in pain. (Prey animals such as guinea-pigs also do this. It's sad as they have often gone beyond the point of help by then and just keel over.)
My friend recently was just taking on a case of a Rottie that was starting to get very aggressive, and on examination he was found to have severe hip displasia. The owners didn't want the op. and also didn;t understand the effect the pain had on their dog although it was explained by the vet and my friend. Shortly afterwards the owners told him off for chewing, a bit too roughly (ie. whacked him) and he went for the woman's face and she had to have 16 stitches. Very sad as of course the dog was put down and that was the end of it. This is one reason why i feel very strongly that dogs should always have a vet check up in such circumstances, especially if the aggression is a change of personality.
Because Toby is such a youngster it's difficult to know what his personality really is I guess! . Of course there is always a chance it wasn't the abscess but let's hope that he will contunue to improve and that both you and your daughter are able to feel confidence about the situation. You certainly sound happier!

Best wishes
Lindsay