Can we require worker to get an eye exam?

Q. We have seasonal employees currently on layoff. One of these employees has been with us on a seasonal basis for 25 years. His health and eye sight are a concern to us as far as safety for himself and others. Can we legally require a medical exam? And how will this work with ADA accommodations? What if he is required to wear glasses and he is found not wearing them? Are there any other issues I need to consider in the re-hiring of this employee? Seasonal positions at the company are never guaranteed a call back. – Peggy, Idaho