Thank you for your question
My name is ***** ***** I shall do my best to help you
Since your son has been living in the UK for the last two years it is for the Uk court to decide which country he should live in.
This is despite the fact that he is Greek and that you have just removed him form Greece to bring him back to the UK.
Accordingly if there is a risk that your ex will try and take hime back to Greece without your agreement you can apply to the court here in the Uk for a Prohibited Steps Order to prevent your son being removed from England and Wales
You can also get help from the charity Reunite
Please ask if you need further help