Further to my last post, I just checked EF for the two flights in question and have a couple of other things to add in case it helps.

It could very wel be that 16:00 and 20:45 are both oversold (16:00 more so). EF is showing J2Y2 on the 16:00 and J3Y5 on the 20:45. If this is the case for the 20:45 flight, then the same day change will not be an option.

I have, however, had success in calling in and having ac make the change for free in advance and not even on the same day (on a flex fare), and they've agreed rather than them having to potentially ask for volunteers in an oversell situation at the airport. Hasn't worked extensively for me, but it has on a couple of occasions, so it might be worth a shot.

Also, if your brother was affected at all with flight delays during this trip, I've also had free changes provided for this reason--even on the return if the outbound was delayed for example.