J.G.
I don't have much by way of advice, but I did want to say I know exactly what you mean. I have noticed this behavor in friends all my life (even as a child this bothered me).
It is very hard for me to grasp how someone can be so careless with "things". When I have tenderly brought up the subject with some people I see doing it, they do act like I am over-reacting and that they're "just things". I feel like it is impossible sometimes to get the point of respect for things, and the person who worked hard to pay for them.
I do agree with some other comments that your husband should be brought into the finances in some way so atleast he can't claim he had "no idea". I do our bills, but regularly give my husband updates:-) it helps.
Keep on your son, cause it will effect how he treats things and people the rest of his life!
Good luck with everything, I hope they try to understand and make improvements.