How to mail a professor informing that I am interested working at different place
These sorts of things are best handled face to face, not by email which is less personal. Go see him. Tell him about the situation and that it is an opportunity that you don't think you can pass up. Thank him for his help and interest. Let him know that your project will need to wait or be abandoned for you to do this internship. 

If he is doing his job properly, then he will support you. And the face-to-face can actually strengthen your relationship for the future. 

You can, of course, say all those things in an email, but it won't have the same effect or impact.