If he had a bone spur or spurs that prevented certain movements, it’s a completely legitimate reason to not be drafted/able to enlist. When I retired ~7 years ago, every recruiter had the same problem; the vast majority of men the proper age were either to fat, to stoned or to stupid to be admitted. I’m sure it’s only got worse since.
A good friend who does a lot of hiring for US Steel has the same problem. Upwards of 90% of the potential hires sent to him from the PA jobs-unemployment office can’t pass a physical and/or a basic aptitude test that any 6th grader should ace. These are good jobs too, ones you can support a family on and have a secure retirement.