My father turned 18 during WWII. He had a leg injury as a child and was rejected by all branches when they saw the scar running the length of his left thigh. He barely had a limp, but the rules were all enlistees must be able to be infantry. He had always regretted not getting to serve.
He would say "They could have let me do something stateside. Somebody had to mail out pay checks and order supplies for the front lines."
You Dad's situation reminds me of a scene in Saving Private Ryan.Namely,the scene in which it's realized that several members of the Ryan family have been KIA.The Captain in the scene walks into his CO's office and we see that Colonel standing and clearly having only one arm.That Colonel surely was injured in action and,after being injured,was still serving honorably to the best of his ability.I think your Dad was right to have the attitude he did.
He had always regretted not getting to serve.
I hope his family and friends were successful in their effort to convince him that he had nothing to be ashamed of...he did his very best to serve to the best of his ability,just like that Colonel did.