Of course not la. 1) Flat has to change owner name after death. If the father writes the last son's name down in his will, then no one can force him to leave it to anyone after his death. The son can make his own will giving the flat to someone else. Not even the law can help you get the flat back.
2) He said he would not marry or have kids. Do u believe him? How can he guarantee that? Some 60 year men marry young mainland wifes and have kids. Who will get the flat then?
3)If he occupies the flat for so long, he should pay rent to the other siblings, not living there for $3000. You can't even rent a 'tong' 房at that price.
4) If he lives there for free for 12 years after his father's death, he can claim ownership of the flat.逆权侵佔.
My best suggestion is not to tell the youngest son that the father has made a secret will that is fair to all children. Best way is to sell the flat and split the money. The will must name an executor遣嘱執行人, don't let it be him. He will change the ownership to himself!!!