I think if your stepson has asked you to tell his father, then I would do it. He is already having enough troubles with his mother reacting badly. He needs someone to unconditionally support him and that is you. Great job so far!! Sounds like you had the perfect response.

I would also ask your stepson to write his dad a letter explaining how he feels. A couple of weeks after you have told DH, then I would give him the letter. Give it time to sink in.

Only your DH can decide how he will react, but whatever you do, make sure you let your DH know that you will always support his son, even if he wants nothing to do with him. Your stepson deserves that much at least!!