I can't tell just by looking at a dog whether my dog will want to play with it or not. I think you have a better chance with two dogs that are similar in size or ancestry ie the two mastiff x sounds good.
I would be a little bit unsure about getting a second dog that would be bigger and stronger than the first dog. This might mean they will fight more about who is in charge at any given time. Or they might not. If Boss likes being boss, and the other dog does too that would be a problem.
The best way to find out if your dog will get along is to take him with you when you go to meet another dog. I know a GSD who lives with an SWF and they don't fight. So it's not always important to match size either.