Well it's not surprisingly that a lot of vets never want to speak of it.http://www.witnesstowar.org/content/view.php?g=s&c=A&...http://www.witnesstowar.org/content/view.php?g=s&c=A&...http://www.witnesstowar.org/content/view.php?g=s&c=A
Have you tried for his military medical or pension records. ?
If he received treatment after the war, and or a disability pension, I believe that the military veteran medical and pension records are held separately from the service records, and should be available via the veterans administration, and should contain his service number.
His Purple Heart will have had a citation document which would have listed his service number.
He would also have been entitled to a Veterans headstone upon application by his family.
Also, I believe that there were certain benefits available via, or to, civilian employers of veterans, in respect of their military service, and so veterans employers were able to apply for and receive certain service docs for veteran employees, and so perhaps a former employer may have some of his service docs in his personnel file, and what about his IRS records, wouldn't they perhaps list his service number from his military days if his militarypay was subject to income tax. ?