That is interesting Rubbertree. The knowledge that Islamic extremism exists is fairly commonplace. He may have got involved in moderate mainstream Islam in order to seek out a group of extremists to join, or he may have been interested in normal Islam and then have become radicalised afterwards.
Either way, I agree nancy75 that he must have been searching for something. I think the drugs analogy works quite well. I know logically that there will be drug dealers in our local town, even though I know nothing about them. If I was interested in taking illegal drugs I could probably seek them out. Similarly, I would have no idea where on the web you would look to access this extremist content, but no doubt I could find it if I wanted to.
The OP was about the parents though. There is a level on which I do feel sorry fo them. They have lost their son to a vile form of extremism and they will never get him back.