Need help with Immigration laws and marriage....?

I have been dating my boyfriend for 3 years now and within the next year we are going to be engaged. I am a US citizen but he is a EWI " Entry without inspection" He is not technically an illegal immigrant yet because he is turning 18 in less than a year. I am planning on filing a fiancee visa form. But I am hoping that be doesn't have to return back to MX before we can go through with a marriage. I need other opinions and advice from people who have been in a situation like this.....what should I do