"He asked for some water to drink. Doctors have given him some oxygen and sedatives to make him rest. This has stabilised his condition," he said. "But I am not sure when he could leave [the hospital]."
Doctors have advised Mahathir to rest and only relatives are allowed to visit him, added Mukhriz.
Mahathir's wife, children and other relatives were at his bedside.
The 81-year-old former prime minister and his wife arrived in Langkawi for a two-day day private visit and were scheduled to return to Kuala Lumpur on Tuesday.
In November, Mahathir suffered a mild stroke caused by a clot that restricted blood flow in one of his arteries, but subsequently recovered.
He also underwent heart bypass surgery after a heart attack in 1989.
Mahathir retired in 2003 after 22 years in office, handing over the reins to Abdullah Ahmad Badawi, whom he has since accused of nepotism and corruption.