One thing that people forget is that someone with a service or ES animal is asking for an accommodation to have the animal at the house. 

And our expert answer is, we don't know.So try to treat this situation as if it were your grandma living in your rental (assuming you like your grandma) and try to be as accommodating, and reasonable as possible. 

Explain your position to them that he doesn't meet your background check standards that you use for everyone, explain that you understand that since he's on the voucher whatever lease they have needs to include him and you can't accommodate that, give them ample notice to move out (perhaps more than whatever is your locality's minimum to show good faith and appreciation for the quiet tenancy they've given you), take the couple week income hit to find new tenants and move on.

Landlords really need to completely understand their right to say no to "reasonable accommodations," and any applicant who happens to belong to a protected class - just because someone's in a protected class doesn't mean I have to allow them if they also smoke and I don't allow smoking, or because they have an emotional support dog that's a pit bull, etc. 

When people make unreasonable or petty requests, I'm not going to be very accommodating. 

There are very few protected classes and they don't include "people who smell like smoke," "people who don't completely fill out the application, even after I tell them it's required," "people with pit bulls for emotional support animals" (hint - emotional support animals require you to approve a "reasonable" accommodation - which means asking you to break a rule of yours - but it has to be reasonable.Knowing the rights of landlords as well as tenants. 

He could not accommodate any time to come down for the keys, so to make his life easy, I left him the keys at the house so he can come to take a look at the property almost 6 days before the initial contract begins.

Also, ADA is not relevant in this situation (to my knowledge) as that speaks to places of public accommodation.

Exemptions include "Housing accommodations in which the tenant shares bathroom or kitchen facilities with the owner who maintains their principal residence at the residential real property.(5) Single-family owner-occupied residences, including a residence in which the owner-occupant rents or leases no more than two units or bedrooms, including, but not limited to, an accessory dwelling unit or a junior accessory dwelling unit."

I want to be sure we go about this the right way if we are to accommodate this guest.