Ebony has a white patch on her chest, which as far as I understand is acceptable but like you say, the difference between breed standards and "reality" is a significant gap for a lot of dogs, especially if they're BYB or crosses. I have to agree with Ripsmom, I'd feel iffy calling your pup either a Bullmastiff or a Presa. I could easily say "mastiff mix" and leave it at that. I assume since he's a rescue anyway you don't really care if he's pure but I can understand the curiosity. The EM x Bull idea sounds like it could be right, but it's hard to classify a mix.
In any event ... he's certainly adorable and looks like he's really attached to you, so excellent adoption! You always roll the dice on a rescue, so I love when it turns out for everyone!
~Marrow