8 year old boy thinking of "sex"

Lastnight my 8 year old ( who turns 9 in three weeks ) came to me and told me he couldn't stop thinking about girls   ( this is the first time he has so much as hinted at this).  He seemed to be uncomfortable and anxious about this but he is a pretty shy child and this was taking him out of his comfort zone.  I explained to him that this is o.k. and that eventually boys start to think that girls are cute.  He was much more comfortable after our talk and continued on his way.  A short time later he came back to me and told me that what he thinks of the girl he thinks of "sex".  We gave him the sex talk last summer because he asked us what it was so he has a basic idea of what sex entails such as it involves two people's genitals touching together (couldn't bring myself to use the word penetration with an 8 year old) and that it is something between two people who care about eachother and are adults.  He is NOT exposed to movies, books, etc. with sexual scenes or inuendos so unless he is getting sexual information from friends, this is the only knowledge he has of the topic.  When I asked him to be specific about what he thinks about when he thinks about sex he was not clear just that the girl makes him think of it.  I convinced him to tell me which girl it is and it turned out to be an older girl who rides him bus ( she's in 7th grade ).  He said he sometimes "watches" her but doesn't talk to her.  I realize that he has a crush on her but my question is if his thoughts are age-appropriate.  I am not sure when he says he thinks of "sex" if he really is or if it's just a situtaion where he feels himself attracted to her.  I let him know that when boys get to be a certain age, they start to have sexual feelings but that he was not at an age where sex or sexual acts was appropriate.  I did not tell him that his thoughts were bad or wrong in fact I told him it was natural.  Is this natural for a boy his age? He is my oldest so I am learning as I go here and I have another son who is now 3 so I would like to know what to expect from him.