I live in Illinois and I was wondering if that was illegal... I don't want him to get in trouble

"The age of consent in Illinois is 17. However, it is illegal for a person 18 or older to commit sexual acts on a person under the age of 18 if he/she has a position of authority or trust over the victim." - wikipedia, age of consent

It's considerate of you to not want to get him in trouble. Don't forget that you are the one who gets pregnant. There is NO birth control that is 100% safe, and only the pill, patch, or IUD have good odds.

"The age of consent in Illinois is 17. ...

So, to answer OP's question, yes, it would be illegal for him to have sex with you. He could end up in prison.

TRUE! I did forget to make that clear.