Well, if he's been sexually active at all -- including oral sex -- then that's why.
The why of people getting STIs is just people being sexually active. And 15-22 year-olds are the people with the highest rates of STIs, so while he's just under that, he's right in that window. Same probably goes for your other sexual partners, who an STI like Herpes -- just as an example -- could be from, too, in the event that's what this is. Just because you may have a sore showing up around this encounter doesn't mean it was from this encounter.
I would strongly advise getting it checked out (and not putting things like Vaseline on it) given where it is and the fact that it's both a little outside and a little inside your mouth, especially since it sounds like you have been engaging in unprotected oral sex, so even if what you have is oral Herpes you, like most people with it, contracted nonsexually in your childhood, it could be something you've exposed sexual partners to without them knowing. I'm not meaning to be scary -- and HSV is very common, so while it's highly stigmatized, it shouldn't be scary, really -- but it's something to get checked out, especially because it's something that you could be passing on to others genitally through oral sex.
Is there a school nurse at your school, by any chance? Alternately, Planned Parenthood has Boise clinics where you could be seen on a weekend and your parents would absolutely not be notified: https://www.plannedparenthood.org/healt ... 2939-91810
If you'd like to talk about how to best prevent STIs when it comes to oral and other kinds of sex, I'm happy to talk with you about that, too. <3