Can exposure to porn at such a young age cause my son to have a sexual interest in pre-pubescent kids?
Yes, is the simple answer I have no doubt.
The longer answer, in my opinion, is that any kind of sexualization at an early age can distort someones sexual boundaries in ver drastic ways as they get older.
So it's not a case of "he saw xxxx, so he's going to do yyyy." It's a mixture of many varied influences that make us the men and young adults we become.
It's good that you are trying to help your family, it's what loving mothers do though, part of the deal so to speak. But I would think very hard about the message the professionals give you, because it might come to the point where you are left with a choice of who to support and believe, and who not.
And their opinions will be foremost in your mind, and I would suggest that if you were brutally honest with yourself you might pre-empt that decision and not have it made for you.
Your son does have problems, 17yo boys don't usually have any attraction towards younger girls, they want to be 'grown up' by 17, and a 14yo girl just isn't cool.
But there is always the 'easy opportunity' to peek, and maybe these younger girls were just the easiest opportunity ?
I would also recommend that you seek and use a therapist of YOUR choice, and certainly one that had the confidence and credentials to deal with his problems.
Why "your choice" ? - because I think your husband is part of the equation, and untill such time as you trust him 100% you should question his motives.
If he has exposed your son to porn it's sexual abuse.
Sadly you are not the first confused mother to ask these kind of questions, and there are many good topics and articles around the site that might also help you, and I'm sure someone like Victor who has a better memory of where to find them will post some links.
I hope we make your decisions easier to take, I can't begin to imagine how hard they must be for you.