Well, I don't have an answer to your original question either. However, I don't think he is fighting purely for US interests. If you believe the introduction of democracry to Iraq to be purely a US interest, I would suggest to you that it has worldwide implications. I also think that the DR receives plenty from the US so that one DR soldier seeking entrance to the US, IMHO, does not warrant any special consideration in the sense that you US owes him something. Afterall, again: 1. the world (which includes the DR benefits from a truly free Iraq and 2. to suggest the US needs to thank the DR by way of special treatment to one DR soldier overlooks the entire present relationship between the two countries.
PLUS its not like the guy is in Iraq for altruistic purposes. The guy is fighting because he joined the army. Going to fight benefits his own career vs not going which is AWOL.
Immigration policy is purely bureacratic and presently as far as I would guess there is no box to check off for Foreign Soldiers Who Were Present in Iraq.