Mental health counseling for your niece would be advisable. If you are suggesting legal action, I think that time, his age at the time, and statue of limitations will work against you.
What would you say is the best option for the man - it is him who's come forward but he's not sure what to do.
I'm not sure that I understand your question. Can you please elaborate?
It's the man who has brought this up. He is worried what effect is has on him now an adult. What's the next best step to take?
Are you referring to the 12 year old boy in your post who now is an adult? What is he worried about ie. legal action, emotional, etc...? I'm not sure what you mean by:
'What's the next best step to take?'
It is best for you to be clear and detailed in your post so that we know exactly what you are talking about and want answers to so that we don't have to go back and forth with questions.