I don't know how to tell my mom that I've been sexually harassed

Two and a half years ago, a guy on my swim team harassed me for about a month. It stopped when my swim coach noticed I was extremely uncomfortable and told him to stop (bless that man, btw). I've been struggling with the memories of it and only just recently told a friend and my boyfriend about it.

Now, this wretched child is enlisting into the army. I'm pissed off since even though he's harassed multiple other girls and is even filed as a sex offender, it evidently has not affected his ability to enlist. I want him to get what he deserves and prevent him from harassing even more girls in the military.

My boyfriend is already enlisted and he can talk to the guy's recruiter to prevent him from enlisting, but he wants me to tell my mom since it can get serious. I'm too scared. My mom and that child's mom are on speaking terms and may even be friends. I don't want the relationship between them to be strained because of me. And it's only harassment and I don't want to make it a big deal. I've been going ballistic over it for the past week the moment I learned he was still enlisting and had a breakdown when I volunteered at a veteran's day event and told several veterans he was enlisting next week. My parents have no idea what's up and think I'm going crazy.

I sent my boyfriend pictures of pages in my journal describing the harassment and I feel extremely ashamed for some reason. I can't even remember half the things I wrote in there and there's this paranoia that what I've gone through isn't real, that I'm making it a big deal, that I'm an evil person trying to ruin this poor person's life.

Overall, I don't know what to do. I don't know how to explain to my mom what happened without making it a big deal. I don't know how to successfully kick him out of the military and give him what he deserves without putting myself out there and him finding out that I was the one who prevented him from joining the military.

Thank you to anyone who took the time to read this.