yep be totally honest, tell DS that because M has died you arent ever going to ee him again, tell him its ok to be sad and to miss M because we all do. if you re explain it to him make him repeat what u say back to him to make sure he heard u correctly. i have a book on kids and grief and apparently at the pre school age they think that death is only a temporary thing and its reversable and they will see the person again.
If he wants to talk about M let him, be warned kids are very very blunt, i asked DD1 where her baby brother was the other day, thinking she'd say in heaven, but she said he's still in the hole mummy. so yeh very very blunt these kiddo's are!

If he goes to day care or kinder explain the situation to them, tell them to allow him to talk about M and encorouge them to ask him questions abvout M but tell them to always end the conversation with M is in heaven (or a star in the sky)

its not easy and i think its an ongoing explaination to them until they fully grasp the concept of death.

i;m so sorry for your loss

Re the funeral, explain to him what happens at a funeral, tell him lots of people will be crying but reassure him that its ok to cry. and ask him if he wishes to be there