I suggest checking the companies on line to read their policy. If you find one which does allow him to rent, then print it.
You also might want to check with his insurance company. If I rent from certain car rental agencies the age and driver policies are different if I use my insurance company's code.
If he has a valid driver's license I do not think the rental company will care, the only major issue they have with is under 25, and it is mostly because of the insurance risk.
Yes, above a certain age can be a limitation. You will have to check with the specific rental location although if you see a restriction on the company's nationwide web site (see the policies or rules section) that is an excuse to be scared off and go to a different company.
This problem is more often seen in Europe as opposed to the U.S.