Are you guys kidding me "is he legal" or "immigrated"...
Did you ever hear of the Spanish American war and the invasion by US forces of
PR back in July 25th, 1898 and then of the Jones Act which in 1917 conferred U.S. citizenship to Puerto Ricans?
Puerto Rico is part of the USA and all Puertorricans are US citizens by birth (like the rest of the US citizens).
As to the last names, its common to use both (I do) and my SS card uses 2, my license uses my dads and my tax return uses both.
All you have to do is to either use his fathers last name or both. No issues with the IRS.