Yesterday he tied the tourniquet on to the young man's legs and wheeled him past the finish line to get emergency help.
A video interview has emerged of the man just moments after putting the victim into an ambulance.
"All you could see everywhere were limbs, I mean ripped off limbs everywhere," he said.
Arrendondo was across the road from the first bomb blast and had been handing out American flags at the event.
In the video interview he shook uncontrollably as he tried to unroll a flag he still held, revealing a large blood stain.
"There was so many people laying there begging me for help ... But I only could help one at a time so I only just helped that young man."
He thought he saw women who had died, he said.
"I just concentrated on that young man and his legs.
"I tied him up and talked to him and kept him conscious and [was] letting him know that the ambulance was on the way.
"I kept him awake and made him stay with me, then took him to the ambulance and I escorted him out."