With income if you don't have it in your screening - I would only use a maximum of 3 adults to qualify. 

In my lease agreement it requires the tenant to notify me of any adults that move in and they're required to get on the lease and pay another $100/month.

I'm in Minnesota.I'm in the process of beefing up my existing lease with additional expectations of tenant behavior (such as not having other adults not on the lease living at the property).

I think that young professionals/adults are interested in living in multifams or sfhs.  

Also want him to have this in case his adult son has concerns and trie sto talk him out of it just due to fear and ignorance on seller loans.

He's a legal adult and living in your property, for which you are liable.

It seems like everyone is required to live on campus unless they are adults or are families.

Any unrelated adults should have to qualify on their own in order to be on the lease.  

We were downsizing, laying off grown adults that had high paying jobs, children at home and clueless.  

I have a single family house at 672 square feet and it houses 2 adults.