I would agree with your husband that your son would learn best from the natural and logical consequences of his being either responsible or irresponsible about fulfilling his promise to do community service. I don't suggest rescuing him in any way from the consequences that he will suffer if he continues to ignore his community service deadline. I suggest, however, that you have a calm talk with him and ask him what his plans are, in detail, to fulfill his community service commitment. After he gives you his plan, I would tell him that you believe that he will honor his commitment and that this is the last that you will speak to him about it.
Leave the "door open" to him for any help that you can provide him in planning out his remaining service hours, while making it clear that he should probably find out from the court what the consequences would be if he fails to honor his commitment on time. I know that it's maddening to watch him behave in this nonchalant manner, but you must allow him the opportunity to succeed or fail in this matter in order for him to learn from the experience.