Confused

Okay, i'm curious to know what is the typical sex drive of a 17 year old male? My boyfriend and I have been having sex since january and in the beginning he used to want to do it at least twice a day and was constantly aroused all the other times. However, now he says he just wants to do it every few days because that is all his package feels like doing it. Normally, I would definately feel it is because he doesn't desire me or i'm just not pleasuring him, but all I hear from him is that i'm beautiful, he loves me so much, i'm perfect, he wants to marry me, and etc. Should I be concerned that his interest in sex has waned? Another thing is that at some point in the day he will feel like it but then later he wont, and when I try to suggest because he is tired because it is later at night or suggest he just try and relax and get in the mood he becomes defensive and says it wont work. Is it really not possible for a guy to get an erection or is it also mentally he may be stressed about not getting one or whatever and is not willing to try? Any suggestions would be nice. Thank you.