I have 2 adult dogs, one I have had for 5 yrs, the other is a rescue I acquired this weekend... my boxer is aggressive toward the new dog( 3 yr old bull mastiff) I need help getting them past this

There seems to be quite a few mastiffs in rescue operations these days, possibly because they are a usually misunderstood breed. People tend to pick them because they are large, try to train in an ordinary fashion,which doesn't work with this breed.

I am saying this in a good way, because this breed can be an excellent choice under certain circumstances, and one is not with another dog in the residence who is already established.

What were your reasons for picking this breed, if you don't mind me asking?

I hope you did expect a certain amount of 'pecking order' when you brought the mastiff into the household. I do hope that the boxer will not maintain this aggressive attitude for long, as it will not hold up with the mastiff, who will eventually want to be top dog.

This is not going to be easy, and there are no hard and fast rules, but I do suggest you put a bit of money into the training and orientation of both dogs by having a trainer into the house to observe and advise.