have his sign a pet addendum to the lease, since the original lease does not allow dogs and now you are allowing the dogs.
Hypothetical. dog bites someone in the house. Who is responsible? dog owner? sure. Landlord who allowed dog to stay in house in violation of lease? I would say yes.
There is a lot more that you have to look out for than just damage to the property!
BTW. to answer your question, this advice is free, but I will charge you for the pet addendum.