It sounds like you didn't refer to S much (or at all) by name when speaking about him. So there could be some element of doubt in J about whether the person on the forms is the person that he wishes to report? People after all can have multiple past therapists. It is unlikely anyone will make a report unless they are 100% certain who the person is.

So one way out of a mandated reporting would be to refuse to confirm the name of the person on the forms is the person you have been speaking about (S).

Anyway: don't let them pressure you into this in any way. If your gut instinct is that this will damage you, you are probably correct in this. This is a difficult situation, take care.