To be fair we are watching this with 20 20 hindsight. He like almost anyone else watching at the time was wondering who this person was who ran on the track. Granted you can probably assume he is his dad but when someone runs onto a field up to an Olympic athlete you want to always be better safe than sorry. He doesn't know what he told the previous guys, and maybe this guy is one of the heads of security/organization for the field.

Plus we don't know what he was saying or thinking. Maybe Derek has said it since, but to give him benefit of the doubt. I wouldn't be surprised at that point if he is trying to usher him to an area with medical. It's easy to think back and be like "oh duh, he wanted to finish the race" but in the moment as an official you are seeing an hurt athlete who is struggling to walk and you're instinct is probably "Hey, you need to get to medical".