It can happen once in a while. Chickens do have saliva, although it doesn't play the same role in their digestion of food as it plays in ours as it helps break down our food as we chew. Chickens don't start their digestive process until it hits the crop and gizzard.
However, sometimes the crop gets over full and it can cause saliva to come out of the beak. You would know something was going on to be concerned about if your rooster started behaving unwell. Just keep an eye on him today and tomorrow just in case, and it would hurt to give his crop a check in the morning before he eats to be sure it emptied overnight as a crop should.