all things considered, what i would do is explain to him that even when being careful, accidents do happen and everybody makes mistakes from time to time but we all still need to deal with our errors in judgment in a responsible manner, which in this case is paying for the lost dvd. i think the cost of the dvd should be shared between you, your husband and your son but because of your son’s age, his part being the lesser portion. maybe 1/4 ??
with you and your husband covering the remainder and no more DVD rentals until this one is paid off. However you decide to handle it though, I think it would be good to ask your son if he feels your decision is fair and listen to any input he may offer. He may surprise you with his level of maturity.