I also know that particularly towards the end of the war US uniforms were often not exactly by the book. I've heard from others who have seen similar issues with rank insignia but haven't seen pictures to corroborate it.
Maybe he made Captain shortly before discharge? Or, as you said, he had two sets with his belongings and a family member put them on?
I will search for something from his records to see what I can find. Ancestry and Fold3 have nothing but I do have his enlistment service number and his officers service number.