18 year old son dating 15 year old

I feel.... Odd about this. I mean the girl turns 16 in a week and that's legal. My son met her at school and he tends to hang out with younger people because he has autism is mentally not on the same level with other 18 year olds so I can see how he doesn't see an issue because he relates to people younger than him and he's closer in grade level. He's a sophomore because he was held back. Like I wouldn't have an issue if she was 16 because 18 and 16 is legal and they date all the time but I feel like this is hitting gray area. I get she's 16 in a week but... IDK .

-And I did speak to her parents and they said since she turns 16 next week they don't care. They have a two year age cut off limit for their kids. I'm not eerie about it.

- I don't think my son is interested in sex yet and that's not me being naive. We are close and he tells he about random boners he gets. But as for sex I'm not sure he's actually interested in sex yet.