Forgot to Pronounce Intentions For His Mother in Hajj
What is the ruling concerning the one who performed Hajj on behalf of his mother and at the Miqat he pronounced the Talbiyah for the Hajj and did not do so for his mother?
As long as his intention was for the Hajj on behalf of his mother, even if he forgot, the Hajj is acceptable for his mother since the intention is a stronger argument, for that which the Prophet (Peace and blessings be upon him) said: "Actions are but by intention".
So if the intention in his coming was for Hajj on behalf of his mother or father and he then forgot when he entered the state of Ihram, the Hajj is acceptable for whoever he had intended it for, either his mother, father or other than them.
Sheikh Ibn Baz