He has a problem with his thyroid gland
its a condition that Marty Feldman suffered from
shouldnt really compare with jacko as this guy didnt have any choice in the matter
with thyroid gland surgery and hormonal treatment it can usually be fixed
this case looks to be from birth so im not sure it can be reversed ?. Its sometimes refered to as cretinism.
I only know this as I have a friend here in PNG who suffered from the same condition, and after treatment in Australia she is as beautiful as ever. I will try to get a before and after pic, its remarkable.
http://www.absoluteastronomy.com/encyclopedia/t/th/thyroid.htm