Hi - my name is XXXXX XXXXX I'm a Real Estate litigation attorney here to assist you.
You can't stop him from suing unless you comply and pay him everything he wants. As for potential success, if he can prove that the problems he now claims were concealed by your son and the other tenants, and he can prove the concealment and that there was no way he could have discovered these conditions earlier, he likely has a claim against them. However, if he has re-rented the property since the boys moved out in May, that gives it a whole new twist because someone else could have done the damage - like a new tenant.
I do think that the landlord may have a tough time claiming that he missed the problems unless he can prove concealment.
I would recommend that you consult with an attorney about the cost of fighting this as opposed to just paying the $1000 demanded. It may boil down to a business decision to spend the money to fight or just pay to get past this matter.
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