The company is aware of his condition because, he has had to have surgery on his eyes from the diabetic condition. He started work about 7 years ago for them with diabetes. Since then the surgery was needed. Even though he lost vision in one eye he has continued working as before, no problems as far as performing his job.
Well, I can only speculate based on the information given. My comments are based on experience in Illinois and from management's point of view.
A doctor's letter certifying his physical ability to perform his job wouldn't normally be part of a recall from LO. However, since he was off on a MLOA for surgery prior to the LO, the request for the letter may be related to his clearance to work following the MLOA. Since diabetes can be progressive, perhaps they have a provision for periodic verification that performing his job isn't detrimental to his health.
If he requires some accomodation to do his job, or other jobs at the company, they may be looking for an OK to recall him to work with no accomodation. Or they may be looking for information on what the accomodation would be for the job opening they might have for him now.
As far as your son's concerns, while I wouldn't put anything past employers these days, it would seem they already have a verification of his good health obtained from him when he returned from the MLOA related to his eye surgery. I'm not sure what getting a more recent update accomplishes for them other than what I mentioned above. Your son's ability to qualify for disability benefits in the future would be based on medical analysis of his condition at that time.
I assume he is non-union. But, is he exempt or non-exempt?
Your suggestion that he talk to the Michigan Employment Commission is good. I'd also suggest he talk to HR and simply ask for an explanation for their request that he provide the doctor's letter. Maybe he'll be pleasantly surprised that there are no unsavory motives. We can hope so anyway.
Good luck to him.