Hunny... you're a wonderful mother... to three adorable children. You always hold their best interest in both heart and mind each and every day.
Your little guy is so smart, deep thinking and fun to talk to. Some children find it harder to come to terms with the knowledge about death... as you of course know... You will reach his mind with your gentleness, caring embrace, listening ear, soothing understanding words, explanations true and told in a simple way... Maybe try let him draw what is on his mind and you can draw too... It might be easier for him to express what he feels doing that and that may open a door for you to walk further guiding him.

I guess it's very individual how to approach a child with this questions and worries... I don't have the correct answers... but I want you to know I'm here to listen and hold you from a distance.