aggression towards our dog

[gilljoh]

Hi

We have just acquired a lovable bull mastiff dog- age four and spayed. Has a good history of being friendly towards other dogs as well as people.Our problem is that particular small dogs- normally mixed terrier types, keep attacking him or a very aggressive. This week one dog bit his lip makiing it bleed. He did warn the dog off in between having his face attacked but to no avail. My husband managed to ward the other dog off by getting in between them in the end. The other dog owner seemed shock and could not get this dog off our dog (max). He was very apologetic so i suppose thats something! I dont think max is submissive as some dogs he is fine with however i am now quite anxious about him meeting other small dogs who seem friendly. Any advice welcome.

ps

he was on his lead when this last attack happened.