All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If your son has reached the age of puberty, then you have to help him maintain the acts of worship, particularly fasting, which is one fundamental pillar of Islam. In your specific situation, you are not permitted to let him drink, even if unconsciously.
Since you know that he unconsciously eats or drinks while sleeping, then you have to keep food, water, and other supplies in some place that is inaccessible for him. You could lock the kitchen, for example.
He is not required to make up for the days in which he drank after the Fajr time while sleeping. This is because there is no legal accountability on the sleeping person. If a fasting person is excused for seminal ejaculation while asleep, then he, by more logic, is excused for doing other actions that invalidate the fast while being in the same situation. It is well-known that ejaculation infringes on one’s fast more than drinking does.
If your son has not yet reached the age of puberty, then you have to look after him and train him in observing the rituals in a flexible and gentle manner. The Prophet, sallallaahu ‘alayhi wa sallam, says, “There is no legal accountability on three categories of people: the insane until they become sane, children until they reach puberty, and the sleeping person until he wakes up.” [Ahmad, Abu Dawood, An-Nassa’i, At-Tirmithi and Ibn Majah with a different wording] The Prophet, sallallaahu ‘alayhi wa sallam, also says, “Whoever forgetfully eats while fasting shall continue his fast, for it is Allah who gave him food and drink.” [Al-Bukhari and Muslim]
Given all these hadiths, there is no sin on your son or on you if you come to know about his drinking after it has already passed. However, a person who sees your son unconsciously taking any of the things that invalidate the fast and intentionally leaves your son be without waking him up is guilty of neglecting the Islamic obligation of enjoining the good and forbidding the evil.
Allah knows best.