"O senin besinci oglundur" cevabi neden kabul edilmiyor??
The "senin" is required here? "Oğlun" isn't enough by itself?
it is fine, but somehow I feel like it doesn't sound good enough without it. I guess there is some sort of emphasis on the son being your son. Sorry, I cannot really explain :) But "O beşinci oğlun" is grammatically fine