Christian Advice: PLEASE!?

I hust lost my best friend in the entire world..on June 2 and he and my son were very close on sunday they hung out together and my best friend (RIP JCB) died monday night or tue morning in his sleep where I and his brother found him dead...that being said..my son 15yr old has this terrible feeling that he is going to die any time now. It breaks my heart because i know it is from the abrupt tragic loss of JCB 19yr old. What can I tell him to make this irrational fear be calmed. He has been saved and is making a few bad choices but he is an awesome kid!! HELP...I cant fix him and dont hae biblical scripture to back anything