This is not like he is 2 he is 18. My vote is easy tell the truth about what happen. Life goes on no matter how much we may not like it death is a part of life.
Added, I have received an e-mail stating my post come across the wrong way. I thought it best to say I am sorry to anyone it made upset. That was not my intent at all. I was not clear it seems on that point. I just mint that it was best to tell the truth because he was older and would understand.