It is a tough situation. At least if you break the news and DH reacts badly your DSS may not see it, then in time DH will calm down and be ready to talk to your DSS.. If DSS lives so far away, your DH may just never mention it again. Y'know, he might know but because it's not in his face he may just live in denial? I hope I make sense.. Although it is more ideal that DSS tells DH himself, I think if you are willing to do it, then do it.. Sooner rather than later as I agree it could cause problems if DH finds out you knew for ages.. Again good luck. It is a very tough situation you are in.
I agree with this.

Your stepson has probably put a huge amount of stress and worry into this and has decided to tell you, not his father, for good reason. I think you should definately tell your DH. It will give him time to get used to the idea before speaking to his son and maybe be more accepting of the idea.

You sound like a great stepmum. Obviously he has lots of trust in you!
Goodluck with it all x