There is a few factors here that could be causing this reaction.
First of all, what exactly IS this reaction you're talking about? Lunging? Growling? Biting? It's a problem that you weren't there for the first encounter. Who was? Did the two dogs meet? Or was his reaction the exact same with the first person that had him, that you're able to wittness?
Do you know much about your dogs history? If he was ever attacked or in a multiple dog household?
Dogs who are more dominant tend to be more reactive when a strange dog is approaching. If this dog is approaching quickly, this could easily cause a reaction out of your male. Not to mention if the dog is coming directly towards you both. (does this reaction occur when your male sees the dalmation across the street as well?)
And that reaction is not neccessarily 'aggression' at all.
Does your dog react to bikes as well?
"If you are a dog and your owner suggests that you wear a sweater. . . suggest that he wear a tail."
Vali (American Bulldog)
Artiro (Cane Corso)