Dumb customers on ecommerce

I run an online website and recently a customer ordered a t-shirt from my website, i sent out the product he needed after calling and confirming his size even after my website has an Size Chart he received the product and called me to tell that the size does not fit him and wants a smaller one so he wanted a replacment, accordingly i replied by saying that i will send you a small size and you can return me the medium one after he returned my old product i sent him the new small t-shirt which he received and now once again he again contacted me that i want size between of small and medium which isnt even possible after explaining this to him he threatened me that i want full refund that we have mentioned on our webiste is not possible in these cases secondly he has not yet returned my product to me as of now and is asking my to pay his end of shipping too and now he has filed a complaint in consumer court against my website someone please help me to understand how to deal with this kinda fool