He was given a sedative in the hospital so that he would not feel pain, and
he remained unconscious for three days. When he regains consciousness,
should he make up his prayers?

Praise be to Allaah. 

Shaykh
‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked this
question, and replied:

 If
it is three days or less, then he should make up his prayers – by
analogy with one who sleeps. If it is more than that then he does not have
to make up his prayers – by analogy with one who is insane. And Allaah
knows best.