Actually he falls asleep very frequently almost everyday and our boss have never concerned or been mad about him. There was only one time that he snored quite loud and our boss patted him so he woke up. But since then he still falls asleep frequently and have not shown any signs to improve. Honestly I've been feeling strange about my boss's tolerating such a person, but I'm still not sure if I should keep pretending not noticing him sleeping, or I should tell him that sleeping at work is really not proper?
There are illnesses that cause people to sleep when they would not normally be asleep. If your boss knows that this person has such an illness and has agreed to a workplace accommodation for it, then he would not necessarily be at liberty to discuss this person's health issues with the rest of the team just as he might not be allowed to discuss any personal issues you have with others. If the boss knows about the sleeping, and in this case he clearly does, then let it lie. Give the person the benefit of the doubt that he is ill and continue with your own work.