Higher sex drive than my bf?

So I'm 5 years younger than my bf (18 & 23), we've been dating for about 4 months and he's turned me down a few times for sex. The first 1 or 2 times was late at night and he said he was tired so I was frustrated but understood. The latest time I was making out with him (planning on giving him a BJ) and he said he wasn't that horny but we could "try." The only time I've turned him down was in the beginning of the relationship when it hurt (he's really big).