Must I fulfill the vow of a thousand rak'ahs I pledged for my son's recovery, or can I expiate it or give charity on its behalf?

Your vow to pray a thousand rak'ahs upon your son's speech is considered a vow of obedience, and it is obligatory to fulfill it, based on the Prophet's (peace and blessings be upon him) saying: "Whoever vows to obey Allah, let him obey Him." Expiation does not substitute for it as long as fulfillment is possible, even if after some time, as the matter is simple. If you are unsure of the number of rak'ahs you have prayed, base it on the lowest certain number to clear your conscience.