First please don't use the Comments feature for followups. Use the Answer options at the bottom of the page instead.
You may have an out. This is covered by:
609.345 Criminal sexual conduct in the fourth degree.
Subdivision 1. Crime defined. A person who engages in sexual contact with another person is guilty of criminal sexual conduct in the fourth degree if any of the following circumstances exists:
(b) the complainant is at least 13 but less than 16 years of age and the actor is more than 48 months older than the complainant
So if you were less than 48 months older there may be an out here.
However, there may be other laws that you could be prosecuted for. The FACT here is that you had no business messing around with a minor. The 15 yr old was not legally able to consent. The fact the mother let you sleep in the same bed could be construed to having put you in a position of authority over him which may negate the 48 month rule. If this goes to trial, a jury is not going to look kindly on you taking advantage of the situation by allowing things to go too far. You were the adult and should have known better and not gotten into this situation in the first place.
He's not trying to "get you for rape". It WAS rape because he was under the age of consent. The question is whether you can be prosecuted for it under existing law. And whether he is having sex with anyone else is immaterial to your case. I doubt if a judge would let that even be mentioned in court assuming there was proof of it.
I would advise you to get a good lawyer, because you are going to need one.