If he truly is how you say he is, then he will probably not want to fill out an application properly, leaving items blank, insisting that you use the credit report him or his agent provide and not one you pull up on your own. Any one of these reasons can be a reason to "fail" him.
You can legally reject him because of poor credit score, poor debit to income ratio, etc. Maybe he has a pet and you don't want that. Maybe he smokes. Maybe the previous landlords wont check out.
Heck, he may not even turn in the application. Many prospective tenants want an app and tell you they like the place but never turn anything in.