His name is Carlos Arredondo, and he lost two sons in the war in Iraq. He came to the US as an 'illegal' in the early 80's. He has since gained citizenship.

After he lost his son Alex to the war, he participated in anti-war protests, and was beaten to the ground and assaulted in Washington, DC during one demonstration in 2007. Link

At the Boston Marathon bombing:

NYT:  ...“The man in the cowboy hat — he saved Jeff’s life,” Ms. Bauman said. Mr. Bauman’s eyes widened. He said: “There’s a video where he goes right to Jeff, picks him right up and puts him on the wheelchair and starts putting the tourniquet on him and pushing him out. I got to talk to this guy!”

The man in the cowboy hat, Carlos Arredondo, 52, had been handing out American flags to runners when the first explosion went off. His son Alexander was a Marine killed in Iraq in 2004, and in the years since he has handed out the flags as a tribute...

Miami Herald: ...He attended other funerals for fallen warriors and logged more than 100,000 miles in his Nissan pickup truck, participating in anti-war demonstrations in Washington, D.C., New York, Boston at the 2008 Republican National Convention in St. Paul. He was beaten at an anti-war demonstration in Washington in 2007....

Carlos Arredondo compresses bleeding artery in the leg of Jeff Bauman, 15 April, 2013.

Flag waving 'patriots' beat Carlos Arredondo while he lies on the ground, after they defaced a poster sized photo of his dead son, a US Marine who died in Iraq. Washington, DC, September, 2007.