You won't win this one on the merits - meaning that you were right and he should have been allowed to board. I would try booking another trip using the same search engine and taking screen shots. If there is no warning at all - the OTA may help you (technically still your responsibility but since the industry trend is to draw attantion to that issue...). You might discover that there us a warning that was perhaps overlooked.

The next strategy would be to see if you can get a credit for some portion of the ticket value . Not what you want - but better than nothing.