Bottom Line - S6-E17
Revealing mistake: While the guys are eating chilli at the table, when Station 51 is dispatched to 'man down', notice that when they're at the scene, both paramedics are wearing their jackets and Johnny is wearing his helmet. We never see Johnny's face in any of the wide shots, because the entire time it's the double with Roy. We only see Johnny alone in a few closeups, pickup shots which were filmed later, and we can see the style of the houses in the closeups do not resemble the houses in the other shots. The real Johnny is back, once they reach the hospital.
Answer: I believe you are speaking of the old man, Mr. Wilson. He's played by J. Pat O'Malley.
Bishop73
Correct, it was J. Pat O'Malley...he also played the grandfather with his grandson when their rocket exploded, and also played "Old Bill" in the episode with Ann Prentiss, where Gage saves the little girl from the burning tree house, and her mother falls in love with him.