Click to Calm (book). Althoguh Gavroche was never that bad, he was dog aggressive. With some of the methods in Click to Calm, he's mostly over it now. I was even able to let him run around with the 8 dogs I was dogsitting (total of 11 dogs including mine) with NO issues after several months.
In the meantime, try to avoid meeting other dogs if at all possible. Walk in the "off times' or go alternate routes. You don't want this to become any more ingrained than it already is.
