My son followed to join on K2 more than 90 days after I arrived on K1 (9 months after), so our dates for ROC are more than 90 days apart which means that he has to file I-175 separately.

However, now the time has come to file his ROC and I am confused about which box he should check. Obviously (or maybe not!) he doesn't check the "married box" as it wasn't him who got married. But then the other box he could check isn't applicable either as it states if the child entered as a "PR" which he didn't, he entered as a K2 and had to adjust status to become a PR.

Which box should he be checking to avoid it being sent back?

Thanks

I believe the rule is that the alien (K2) must send a separate I-751 if he adjusted status more than 90 days after the K1. Is that the case?

You would check Part 2 "b"

Yes that's the rule, but the boxes he can choose from to check on the petition don't seem to allow for that scenario.

It asks whether he is either the one married, which he isn't or a child who entered as a PR who is filing separately which he didn't. He entered on as K2 and then adjusted to PR.

All the other possible choices relate to marriages ending for some reason, so none of those are correct either.

I thought I had this whole thing sussed, but now I'm not sure which one to check. Don't want it sent back or rejected and have to re-file, but it doesn't seem to make sense on the form itself.

His PR is conditioned upon his mother's PR. But it is clear that he must file alone and is unable to file jointly with his mother due to the fact that his AOS was not approved within 90 days of his mother.

Heehee, I guess these forms are not as accurate as they should be, but as far as your question is concerned, I am sure he would tick "b"