Gavin,
Got to warn you, a 17 year old protag in a YA novel is going to be a tough sell. We've just been through that. Had a lot of positives on our FORESEEN concept, but the 17 year old YA - who happened to be a girl, was a show stopper. Yeah, you do hear that this is acceptable but in our experience, most YA agents are going to pass.
The reality is, there are very few books out there with protags between 17 and 25. Whether you agree with this or not there is a reason for it: Westerners this age are in college. They are busy and not reading all that much. You may counter by saying, younger kids would read it. And you'd be right. But most agents won't. Sad but true.
If there is any way you could change the age of your character to be younger it'd make your book a lot more appealing to most YA agents - regardless of what they might
tell you. You should strongly consider this now, before you put a year into something that is going to be an extremely tough sell in the YA market.
That said, is there any way you could recast the book as SF or Fantasy. Or even paranormal? You didn't say anything about plot so it might be a non-starter, but if you can, your protag's age is much more acceptable in these genres.
Make sure you educate yourself about genre. I thought I had mine right in YA and marketed my work that way. Didn't get zip. I finally realized my writing was closer to SF. I started marketing it accordingly and got an offer for a three book deal within six weeks. I don't know if I'll take it as the deal isn't great, but it definitely shows that proper marketing goes a long way. I recently wrote a blog post on the subject of knowing your market. You might take a look at it.
Know Your Literary Market.
I'd think long and hard about both of these points. If you're going YA, make the protag at least a year younger. If you want the protag older, see if you've got the right genre. YA isn't just about having a young character, it's about voice. Again, see the blog post.
Good luck and let us know what you decide.