I'm going to be somewhat brutal in my response, but I don't mean to offend you. You want an answer and one of these may be the it:
1) Your boyfriend is actually gay.
2) Your body has significantly changed since you first met
3) Your boyfriend went on medications that suppressed his sex-drive
4) He's found someone else
At your ages, it's not normal.
It's possible, I suppose, that 3 months after you met he suffered a testosterone depletion, but I think not.
However, did he go on any medications 3 months after you met?
I don't know what you've found on the internet, but you're not crazy and there's no reason for you to not expect a more engaged sex-life.
He has to be honest with you. "It's normal" is not being honest.
My advise is that you put it to him that he needs to be honest or you will assume the worst and leave him.
I know the "leave him" advice is never well received, but if this is irreconcilable, it won't get better and you will only be wasting precious months and years of your life.