Does he allow other dogs onto his territory? How does he deal with older dogs, or with other males?
I've been considering adding another dog to my household (I have an OEB x Boxer who is a ~3 year old spayed female) and have spent the past year seeing how she likes other dogs on and off her own territory, and how much challenging there is before she accepts a new doggie friend. I have seen her get in fights when it is in her own turf, but will eventually accept a new dog if it does not challenge her too much.

With any sort of Molosser breed, you will want to get a female dog of a similar size because they will be less likely to fight than two males, and will also be less likely to hurt each other playing. What about a big American Bulldog or AmStaff?