How do you prepare your child for the death of a playmate?

I personally, if I were your son, would want to be able to enjoy what times left with my cousin. Honestly, that boy is not going to want his life to end with his cousin acting different because it may be his last summer. Besides you never know but he may be strong enough to live longer because doctors can be wrong. I think that your son should be able to be a kid just like his cousin should.