Hi Faye:
If you have his picture in Union army uniform, then you should be able to easily find his Compiled Military Service Record at the National Archives plus any information on any pension application that he made for a Federal pension. Check out the National Archives website for "how to" information: http://www.nara.gov/genealogy/civilwar.html
These Union army records will tell you when he enrolled in Federal service, his age, what unit he served with, etc. There were some 5,000+ white Louisianians who served in the Union army during the war.
Based on the pooled information we have so far, I still think your Phillip PAUL [Rapides Parish] is not Private P. W. PAUL [Bossier Parish] who served in the 9th Louisiana Infantry.
I would very much like to know what you find out, so please keep me posted.
Hugh