Continuity mistake: During the structure fire, after the parents realize their son ran back into the burning house to get the dog, when the father tries to go after the boy we can see the man's wearing a striped T-shirt under his robe, but when Roy rescues both the boy and the dog, the father is now wearing a white T-shirt under the robe.
Continuity mistake: After 51's guys rescue the man from under the fallen engine, when the ambulance leaves we can see only the name Mayfair is printed on its side, but en route to Rampart and when Roy has the ambulance stop and then go, there's a red cross printed on the surface above Mayfair.
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.