Maybe put a webcam on a stick and work it into the cavity behind that wall to see what you have. 

(I had a bout of adult onset asthma and I don't want anyone else to go through that)

But if it's not reasonable, you need to get it to market rates.Basically you want to talk to the person adult to adult.

It's not proper remediation, it's illegal to not do proper remediation and if you think selling a remediated house might be a stain on your reputation, try selling one where you claim to have remediated it but haven't (which is what this advice is).Meth is nasty, nasty stuff causing advanced and rapid neurological damage in children and pets and slower, but no less advanced neurological damage in adults.  

Their adult sons stay in one room when in residence and sometimes visitors are placed there.

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.