Aggressive Great Dane?

I have a 7yr old Great Dane, Rudy, who has over the past few months become pretty dog aggressive as well as aggressive towards children that are basically at his eye level or smaller. He has attacked my other 2 dogs on 2 different occasions, luckily we were able to break it up before it got too bad. (These aren't new dogs to our home) Rudy has been to the vet to rule out any health issues as well as having his vision and his hearing checked. All is well on that end. We are going to be working with a behaviorist starting next week but in the mean time, I am just looking for some opinions on what everyone else seems to think could be the issue. Rudy has never had any behavioral issues in the past and the vet seems to think maybe he is just getting less tolerant in his "old age"....Would love to hear some opinions.