He should've never been left with a computer with internet access. It would be akin to giving a child a TV with dvd player and p 0 r n dvds and hoping he would not discover what's on them.
In any case, a 9 yr old does not develop such an uhealthy interest in sex on his own; nor does he find p 0 r n on the net on his own. This is very curios. Someone must've helped him, or he found someone else's files on the computer.
Hence, be dilligent in finding the culprit - it may well be his grandfather or someone else you trust; I've talked to people who were sexually abused and the abuser was usually low profile family member, family friend, always very trusted (an uncle, grandfather etc), or friend - even a friend of his own age who was abused himself. Any time your son is not under your own supervision, you have to take into account the possibility that he is abused.
Also, the abuser will make him promise not to tell, else who knows what kind of grave punishment will follow. Or your son may be just too ashamed.
Again, I repeat: he did not develop interest in sex so early all on his own. You have to find out how it happened, and not by asking direct questions. Rather then asking "did anyone show you his peepee?", better to ask so that he doesn't know where it's all going. "So, how was it with uncle Donald? Was he fun?" then "What did you do first?" and when he sees that you have genuine interest in his "fun", he will probably let you in on stuff he would not normally tell you - the urge to share interesting bits of info will be stronger then his fear or embarrasment.
ps. It is also possible that he got introduced to sex by accident - maybe by spying on you and your husband through a keyhole (if he heard noise or sensed something "strange" was going on), or by seeing things on TV or in magazines.
pps. Could he have been abused by a female? I would not be surprised... how else could he have developed an interest in oral sex with a female? See if there was a female that he was alone with and start asking nice questions...