tenant says house has mold/air quality issues (philadelphia)

[Robert Hastings]

a tenant in one of my rentals claims the house has mold issues.  this home is probably 100 years old like most of philadelphia homes.  i have been vigilant in all repairs at the house.  the house, like most, always need some TLC.   he says he and his lady have allergies and so do the kids. allergies to the point they wont open the windows.   they have always had allergies.  but he mentioned the word mold and i checked the house and there is a small patch in two different areas that were addressed.  maybe a total of a couple square feet.  i dont even know what they were.  but the tenant pointed them out to me and i addressed them. i sprayed them three times over the course of three days with bleach and painted them with mold killer from depot @30bucks a gallon... they were on bricks in the basement and in the walls above the drop ceiling.  they spots were isolated to one area.  i checked the rest of the home and there was nothing visible.  i treated the visible areas (as they cannot be removed).   

i went a step further and we spoke about the insulation in the home.  the insulation is just sitting on the drop ceiling tiles all throughout the second floor.  the insulation is old (and it is). my tenant is the son of the seller.  my tenant has been in the house since he was born.  to ease his concerns for allergies and air quality, i paid to have the old carpets in the first floor removed (they were there for 20 years)

i am removing all the drop ceiling tiles and insulation in the second floor and installing new sheetrock throughout the second floor (close to 80 sheets or rock). total cost around 4k! i even purchased two $150 air purifiers and gave him a third to be placed throughout the home to help with air quality. he wife and two kids live there, again, they all claim to have allergies and they think its the air quality.

  the tenant has been with me for three years.  he renewed his lease.  he signed the lease acknowledging the home is in good condition.  i even waived three months rent to compensate for the inconvenience of air quality and having contractors in and out on top of paying several thousands to remedy what is an out of date old home morphed into air quality issues.   i offered him a free place to stay at another home of mine that is vacant (it doesnt have the modern conveniences but its a free place to stay.

i dont know if the tenants are extremely sensitive to dust and airborne particles?  im not sure how far i should go?