teenagers and sex

My oldest boy is 17 (my nephew - he and his sister have lived with us for the past ten yrs, after their mom passed away). Recently, he told me that he had sex for the first time (twice in the last month) with his girlfriend, also 17. We are very close and he tells me (pretty much) everything. When I asked him if it was everything he had expected, he said "no". I explained to him that sex takes practice, like anything else, to get good at it and that the first few times it's awkward, nervousness, etc. so it  won't be all fireworks and whistles at first.  He agreed and said that he was sure that was it. I got the distinct impression from him that it didn't really go well at all. When I asked him why he didn't feel it went very well he said there are no problems getting an erection or anything like that. It must have just been nervousness, inexperience, etc.  He told me that he did not feel that he or his girlfriend were ready for that type of relationship and that they wouldn't be having sex for a while until they were both more mature and ready(his girlfriend has some issues she is seeing a pcychiatrist about and he doesn't feel that he or she is ready for this serious of a relationship). The boy does not lie as a rule so I trust him when he says that they will not have sex for now. The conversation turned to masturbation as an alternative to sex and I suggested he stick with masturbation to "get by" until he and she are more ready for it. He told me that he doesn't really feel any urge to masturbate very often and may only do it once a week, if that, sometimes as little as once every two weeks. On further discussion he also told me that he doesn't really get "horny" or "lustful" feelings even when he wakes up with an erection.   Now, it has been my experience that the average 17yr old does not stop having sex once they start (who does!) and that the average teenager masturbates A LOT more than this and generally has "lustful" or "horny" feelings much more often. I am asking if I should be concerned about his sexual health or if this is normal. Also, he has adhd and has been taking concerta for the past 7 years until recently when the pediatric pcychologist he has been seeing took him off the meds as he has been doing so well. He continues to do well without the meds. I am wondering if the meds may have had an effect or if I am worrying about nothing. I realize that people have different sex drives, etc. and that it is hard to categorize somebody in an "average" range. I am absolutely positive that the boy is not telling me everything and that he is holding something back, like maybe he IS concerned about it or something,however, I don't want to push him in to telling me before he is ready so I would like some advice on this before I talk to him again.