Do you have his marriage certificate? it may contain Regimental number,
So you have his Regiment, his date of birth and his name and he served in ww2 or just after,
why not try to submit that info and see what transpres.
Are there any photos of him? they may contain info.
Did he qualify for any ww2 medals? It may be beneficial if you ask your friend to search family documents, letters from Germany and family photos, letters home from military areas usually contain the senders rank and number.
Good Luck
James