My son is now 8 years old, but his mother, my ex-wife, refuses to hand over custody to me.

Answer

Now that your son is 8 years old, custody is your right. But in this country, if the mother has no regard for the Shariah, she will not accept the ruling of the Shariah. If there is no valid Shar'i reason, it will be haraam for her to deny custody to you.