My boyfriend is Turkish citizen but he has a green card as his mother is a teacher. Does he need a visa to visit me in the UK?
Of course, the only thing that matters in this case is nationality. US green card in this case only serves as a proof that he's residing in US legally.
This page explains whether you will need to obtain a visa before you come to the UK as a general visitor.
You will need a visa if you:
- are a visa national (see 'More information' below); or
- are stateless (you do not have a nationality); or
- hold a non-national travel document (a travel document which does not give you the nationality of the country that issued it); or
- hold a passport issued by an authority that is not recognised in the UK.
The long list of "visa nationals" contains Turkey.
Or you can use "Do I need visa?" form, which yields following result: