I give in and let him have his spacious litter box with cover and lots of cat pine back last Saturday. He immediately let go inside his litter box, both pee pee and poo poo... and boy, he is so happy after coming out from the litterbox.

I will try another approach by first to take his litter box cover away then slowly increase the height of his litter box... I think will take a very long time... sighs.