Hello Hakluyt. Welcome. What kinds of things does your son take from the house?
My son who is 36 takes food. A number of years ago when he was 25 or so, although he denies it, he took an IPAD type of computer.
What about putting in cameras inside the house and in the garage? This would not be specifically directed to your son. He would still have free entry to the house. The cameras would help him stay accountable to himself. And the cameras would not be directed towards HIM.
Maybe your husband's resistance is because changing the door lock and garage pin are specifically directed to your son. He does not want to hurt your son or damage their relationship.
I am remembering many years ago I asked my mother if I could stay in her house while I visited her city. I think she was away. I was already over 50. She said no. She said she didn't want anybody in her house. I was crushed. I lived far away, I did not have a key. My sister lived nearby had had a key. If stuff was missing that was between them. I got painted with the same brush. Being denied entrance to my mother's house just hurt me so much. All these years later, my mother long dead, it still hurts.
Maybe this is why your husband is so adamant.
As far as your son taking things, if it's food, that kind of thing--that's way different from money, jewelry, power tools, etc. To me, it would make a difference.