If a person is kept on ventilator, can he live forever, or will there be a situation that his soul will depart even when he's on ventilator?

No, a person cannot live forever, even if kept on a ventilator. Although the ventilator can sustain a person's lungs indefinitely, it is unable to keep the other vital organs functioning and nourished in order to maintain life. Additionally, even with all of the advances in modern medical technology, no one has been able to “cheat death” due to natural causes or illnesses; an individual’s soul will eventually depart when their body can no longer fight off disease or injuries. The only hope for immortality lies in scientific breakthroughs that don't yet exist; as such, it is impossible for someone who is being kept alive through use of a ventilator to live forever.