Originally Posted by Lux Flyer

Trying to understand this completely so a couple different responses depending on your situation from most to least likely situation I think you're in. Whats confusing me in your question is the "just flew the outbound". Was that you? Your son? Both of you?

Option 1) Your son is flying his inbound. You have a separate round trip reservation where one of the flights is to accompany him (your outbound segment), and then flying back on your own (your inbound segment/return). Your trying to change your inbound. If so, then the agent will be able to manually process the change for you overriding the error message because your inbound segment isn't actually the accompanying segment, even though the PNR is linked.

Option 2) Your son is on their own PNR. You flew with them on the outbound and were going to fly with them on the return, but now want to change your flight to fly separate? If so, they will need to unlink your PNR in the backend, process the change for you, and you would need to pay the UNMR fee for your son.