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?
No implications. This is a certification to the French bank that he is a US tax resident. It stems in the US FATCA legislation.
Your son will continue filing his tax returns and paying his taxes (if any) in the US regardless of this form.