My step son is slandering my hubby and i is there anyway to stop him?:
First of all, what your stepson is doing is libel, because it is in written form. Slander is oral, which he may also be doing, but since he's sending letters, it's libel.

If you're serious about stopping him, start with the police. Libel laws vary from state to state, it may not be a criminal matter in New York, and they will be able to tell you that. 

If it is a civil matter, speak with an attorney about bringing a case against your stepson and what a reasonable expectation of the outcome might be. It might not be worth the time, effort, and money involved if your stepson has no assets to lose, etc.