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Hello and welcome! My name is ***** ***** I will try my level best to help with your situation.
From a purely legal perspective, if son wasn't listed as an occupant on your original lease, then only you and your husband have the right to live there. The lease determines who has the legal right to be on the premises. You are basically asking them to modify your existing lease so they can essentially require whatever terms they want in order to modify the lease to allow son to live there.
However, if the lease isn't limited to just the two of you as an occupancy restriction, then you could have son stay with you. For example, if your lease had an occupancy limit of 3 or 4 people, then you wouldn't have to get their permission to have son stay with you temporarily.
As an aside, I have also been a landlord for over 26 years.