Our 21yo at home son is unaware we learned he's obsessing over fetish anime/stories. he suffers from depression and is very isolated. should we tell him to stop? we are concerned about his reaction to embarrassment and finding out we know?

Please get proactive and find professional help ASAP. This may require assertiveness from you, but his emotional issues need to be addressed without delay. Find a local psychiatrist TODAY, consider cognitive-behavioural therapy, talk with a professional to get support prior to talking to him. This is complex, but delay could be devastating.

It's important to get professional help for him. In my mind Cognitive Behavior Therapy is a good place to start. The therapist should be experienced. Even though he may be embarrassed that you know, it will also probably be a relief to know that you know. Keeping a secret has the paradoxical impact of making the obsession stronger because he has to keep thinking how he's going to keep it secret