How Can I Explain A Persons Death To A Child?

Yesterday my boyfriends grandfather passed away after battling Cancer.

I have lived through enough of my own friends and families death to know how to feel and react, but for him it is a somewhat new area.
I know I need to be there for him, but what happens when he won't let me? Lets face it he is a man and at the best of times keep their feeling hidden away and pretend everything is fine, when we all know it isn't.

I am having a hard time trying to explain that he's better now that he is gone and that is really true but knowing that and not being upset are two different things. My boyfriend is glad he's no longer in pain and knows it is best for him to be free. He just can't deal with what he's feeling and to add that on top our three year old has no idea what is going on.

We told L that his Great Grandfather had died and he thinks that he can come back another day which is normal and I know that, but for someone else to be grieving its a lot harder for them to hear that and not get upset.
I have already made the choice that L is just to young to deal with everything that we will have to go through the new few weeks and its best he not attend any gatherings for his Great Grandfather.
I asked my boyfriend what he felt about it and he said to me "I guess if you think that's best".

Have any of you dealt with trying to help an adult deal with death for the first time?
What are some tips that could maybe help me through this difficult time?

Do you think my choice for our son was the right one or should I let him go to funeral?
What age were you when you lost your grandparents? Are they all still alive?

I was 16 when my dad's mom passed away after an accident and then 6 months and one day later my dad's father died as well from cancer.
My other grandparents are still with us and I am so thankful for that and them to be here.
But I have lost a lot of people in my 26 years including my best friend 5 years ago this march.

If you have any ideas on how to help explain death to my son that would be wonderful please.