If someone in the United States did what you do, they would be subject to a Fair Housing complaint, unless they fit within the size limitation exemption.Neither the codes in the US or Canada state that mentally unbalanced applicants have a right to  preferential treatment.

If he gets special treatment from you then he will demand it from the next landlord.

Call me an idiot, but I promised his mom I would take care of him when she was in her final days of fighting her cancer in 2008.

Had a potential tenant ask me if I did monthly pest control treatments on property??  

Just disrespectful treatment.

I can't think of any other reason for that, except they expect that to be difficult = PITA.Maybe not, but in my life experience, you're way better off to deduct the reason is because the tenant is a PITA than some benign reason that equates to the tenant not being a PITA...And to anyone who is tempted to say something like, "Well, maybe the tenant is great, but the seller has cancer or whatever ----- my answer is, really?  

So you can't charge additional for an ESA, but surely you should be able to have an addendum, OK call it an animal addendum to require general things like the tenant is still responsible for cost of any animal damages, or flea/tick treatment of the property when they vacate, or keeping up with vaccination, must pick up after the dog etc...on the other hand, let's use "pick up after dogs" as an example...if the tenant is blind and has a seeing eye dog, how would the tenant go pick up after the dog? 

The place has been treated, preventatively, on a regular basis since the heat treatment years ago. 

My daughter paid for a  monthly service in CA, and figured it was more cost-effective, because if there was a need for a special treatment (ants are bad in that area), any special treatments were free - for people with the monthly service.

Here's a version from Wikipedia:There is a considerable body of case law concerning the proper treatment of a scrivener's error.