Secual Curiosity? or Sexual Abuse?

My husband just informed me of an incident while he was younger. Supposedly he was introduced to pornography which made him very curious at a very young age of what it was that was taking place in those pictures. I guess from the ages of 9 to 12 he occasionally asked his younger sister (7-10 years old) to perform those things in the magazine.
He has struggled with depression and he finally opened up saying that he believes this experience is the root of his insecurities. He says he feels so guilty now knowing better and that is was wrong, but at the time he did not understand the ramifications of it.
Supposedly they never had intercourse, but ended up touching each other a lot, and I dont know for sure, but my guess is that stimulation was involved sometimes.
Anyway, I do not really know how to handle this situation or help him. I feel naive to the whole thing and dont know if I should think that he was acting on curiosity, or if this should be seen as more intense and he should seek help?
What should I do... or what should he do?