We had a horrible encounter today, with my Ddog being the aggressor 

We have only had him for 2/12, he has been v good in his encounters with other dogs, friendly, interested and very happy to have a play, never previously aggressive, seems equally happy with other breeds and sizes.
Today he had a right go and a poor little shitzu or lhasa apso (I am not sure) who from the start was very very submissive: slinking away, tail between legs, rolling over and showing her belly, her owner said she was 'timid'.
As my boy had met little white fluffy dogs frequently before and was always fine with them I did not see trouble coming. I also thought that submissive behaviour would defuse the any brewing aggression??
Ddog does not realise that he has just lost lead-free freedom.
Any suggestions?