Hi.
My 6 month old isn't a Bullmastiff but I have had concerns about him and other dogs. This is because due to this breed makeup he may not get along with other dogs once he is mature, and he is likely to be highly territorial.
He started out very timid about other dogs – crossing paths with them, or even hearing them bark in the distance. This surprised and disappointed me.
Lately, because we have been socializing him in the form of play times with dogs that we trust are hopefully not going to be overly aggressive with him, he has really become a lot more confident with dogs and his love of play has come out.
The reason I have worked hard to get him to be around other dogs is that I don't want him to actually be dangerous to other dogs when he is mature. I am okay if he does not get along with dogs but I don't want him to think he has to attack or kill them.
Have you thought about trying to gradually socialize your Bullmastiff with other dogs, and work toward having him getting used to playing with them or at least coexisting with them?
The other factor would be for you to make sure you are training him and getting him to see you as Alpha. This should cause him to defer to you for how to act when encountering another dog. Typically, if a dog does not respect its owner it takes charge of encounters with other dogs. It may feel it needs to protect the owner from the other dogs. Also, even if a dog does respect its owner, if the owner does not take charge, for instance by issuing the command to be quiet / to not lunge, or by putting the dog on a heel command or a sit-stay or something the dog may default to taking charge of the situation by barking/lunging.