光头 and 秃子 both mean bald.
you will say 光頭 instead of 禿子 when you wanna describe someone without hair,
禿子 is a more impolite saying, it's rude to say that.
sometime you will avoid using 光頭 either, when you talk to him directly
ask a question like 你為什麼是光頭? (why are you bald?)
you can replace it as 你為什麼沒有頭髮 (why don't you have hair)
in this way you will make it less offensive,
though it's still not proper to ask something like this : )
He is bald ====> 他是禿頭
I realize he is bold, when his wig fall off.
He is going bald ====> 他正慢慢禿頭中
I can see the skin under his hair, he is going bald
他是秃子。他"是"光头。 these will be correct
or you say 他有個光頭
(I forget to send the answer out at first place, some information might already mentioned by other people, you might read the same thing againXD)