First, when you hide your money and do not declare it, it becomes a black money for you and it will hard to cash out (if it is in large amount) as the authorities will as for the source of that money.
As far as if he goes in Dubai (or any country which has no tax on bitcoin), cashes out, develop a business etc, that is ok but when he will move back to his won country he would have to pay his taxes on the amount which he currently holds as per his country law. There is no way you can escape from tax.
Secondly. is there no tax on bitcoin in Dubai and when you convert that bitcoin into fiat, won't the Dubai government charge tax or is there no tax on fiat money too
That's strange by the way.
The @OP is looking for a way to reduce taxation, not hide it and find a illegal way in order to not pay tax. As far as if he goes in Dubai (or any country which has no tax on bitcoin), cashes out, develop a business etc, that is ok but when he will move back to his won country he would have to pay his taxes on the amount which he currently holds as per his country law. There is no way you can escape from tax.
Secondly. is there no tax on bitcoin in Dubai and when you convert that bitcoin into fiat, won't the Dubai government charge tax or is there no tax on fiat money too
That's strange by the way.There's a way he can hide paying tax by creating a business in your country and the other one is on a country where it's tax free. Although you need to lie since you need to create a fake purchasing and selling evidence, so you can manipulate the financial report to set the maximum amount where you will not get taxed.
Maybe you still need to pay small amount money for money changer or tax, but it do greatly reduce paying tax.