We have a 12 1/2 month old english mastiff and were looking at getting another puppy?

My dog (little) bear is a 12 1/2 month old old english mastiff. He plays with my cat a lot but she only weighs 10 pounds and he weighs 140. He's the best behaved dog we've ever owned and i made sure to socialize him with other dogs, he has encountered about 15 dogs so far and the worst reaction he's had was being scared of a yorkie down the road. He doesn't bite and he guards our house very well. He is very affectionate but I wasnt sure if he would still act this way around a much smaller mastiff pup or if he would play with it like he does with our cat. Has anyone bought a mastiff pup and already owned a mastiff and if so did they get along?