I'm not sure that I can give clear rule. Perhaps this: We use "who" when the pronoun stands alone. We typically use "what" or "which" when it is used as an adjective or is itself modified.
Who can say that he is not afraid?
What man can say that he is not afraid?
Which of you can say that he is not afraid?
We wouldn't say "Who man can ..." It's "what man" or "which man". But we also wouldn't say "What can say ..." if we were talking about people.
But if the qualifier precedes the pronoun, we use "who". "The man who can say ...", NOT "The man what can say ..."
In your example, there is a qualifier following the pronoun, so we use "what". An alternative way to say the same thing would be "Someone who looked like the oldest boy ..."
Now that you bring this up, I think English is rather confusing on this point. :-)
Note that if we are talking about a part of a person or some attribute of a person, we normally use "what" or "which". "Which leg did Bob lose in the accident?" "What emotion did you feel when you heard the news?" Etc.