First, I'd agree with the "if you have to ask, then you already know the answer" sentiment.

Secondly, I also have to agree with the "why is he going for someone who is underaged" idea.

When I was 17/18 I also dated someone older, at the time I really thought nothing of it. And even now we are still amicable and talk now and then. It was a great relationship and I do not have any regrets. BUT, now that I am older (25) I really have a different perspective on the age gap.

As a 25 year old, would I date someone who is 20? Heck No. When I was 20, would I have dated someone who was 17, even more of a heck no. The fact of the matter is, you may not realize it now, but with life experience you'll come to realize that 17 to 20 is a big difference, not in age, but in life. You two are bound to be mentally, emotionally, and socially in totally different places in life. And when I meet a guy who is in his 20s and dating someone much younger than him I always wonder what his issue is. Because usually there is one, small as it may be (with my ex it was that he suffers from very mild depression and thrives on feeling emotionally connected to someone/needed). If there isn't an issue (something like stunted maturity) then the guy is often taking advantage of the mental age of the girl.

Don't want to get preachy here. But I'd say wait it out. It's your senior year of high school! Find a guy who can enjoy your senior year WITH you!