Seems to me that he abrogated his rights as a citizen by going to fight against the US. I hope he rots.
There are seven basic ways in which US citizenship can be lost:The ABCs of ImmigrationBeing naturalized in a foreign country, upon the person’s own application made after reaching 18 years of age;
Making an oath or other declaration of allegiance to a foreign country or division thereof, again, after reaching 18 years of age;
Serving in the armed forces of a foreign country if those armed forces are engaged in hostilities against the US, or if the person serves as an officer;
Working for the government of a foreign country if the person also obtains nationality in that country, or if to work in such a position an oath or other declaration of allegiance is required;
Making a formal renunciation of US citizenship before a US consular officer or diplomat in a foreign country;
Making a formal written statement of renunciation during a state of war, if the Attorney General approves the renunciation as not contrary to US national defense; and
Committing an act of treason against the US, or attempting by force or the use of arms to overthrow the government of the US. Renunciation by this means can be accomplished only after a court has found the person guilty.
Although, as the website states, it is hard to prove and could be argued on semantics. I say bring him to the US and prosecute his sorry carcass, although lethal injection is way too mild for my taste. Maybe he'll have a fatal accident on his way back or suddenly come down ill, lead poisoning would be nice : )