My son is opening a bank account in France. He was born in the USA, while we (English Parents) lived there for a few years. For the last 20 years (since age 5), he has been back in the UK, and is dual nationality UK & US. He has both passports. He lives & works in the UK, and pays UK taxes. The French Bank are asking him to fill out a W9 form. Are there any implications for this?