yea, its normal. not ALL boys have them. it typically accompany having a dream in that you are having sex. but if you dont masturbate then you can have one for what seems so apparent reason, it happens that was because of pressure of built up sperm in the testis.
I'm 17 going on 18 and haven't had any, doubt i will. Probably depends on how much you masturbate or not or maybe your dreams. I hardly ever dream about sex so maybe thats why i dont get them.