How to write a short letter to a dying uncle?

i'm going to write a short card to my uncle with pancreatic cancer who is going to die in half a year. i'm not extremely close to him because he lives in florida while i live in manila, but he's the closest thing i have to a father, and i really don't know what to say, but i want to make it light and funny because he's very witty, and funny and practically a genius. i also want to say how he's lucky that he won't have to go through the world war III in a funny way, and as subtly as possible but i don't know how to phrase it. (should i put that?) but anyway, please what should i say? oh and i'm 14. :) thank you so much, i would really appreciate it if you'd answer. :)