/Blow job/

So i have been talking w this guy for 2 months and he wants me to go down on him. I really like him and i want to. But he told me that he is very girthy, like fat. Not long just fat. Im scared that my mouth cant open that much. I want to take all of him but i dont know if i can open my mouth that big. Is it impossible to do it? Like deep throat

.. please help a sister out