My sympathies for the loss of your brother. Death is never easy.
My father passed away unexpectedly when I was 7 so I have a different perspective to share.
First, be honest but keep it age appropriate. Tell him enough to give him a basic understanding of the circumstances but leave out the gory details. Make sure he understands he is safe and what happened to your brother won't necessarily happen to him. Tell your friends and family how you explained it to your son so they don't tell him something different.
Make sure his teachers and the school are aware. That way they will react appropriately if he has a melt down in class. Ask them to let you know if they notice any changes in his behavior.
Let him see you upset but let him know its okay if he wants to talk about his uncle. My mother would cry whenever I mentioned my dad and it freaked me out to think I was causing it. I stopped talking about him.
Tell him what to expect at the memorial service. Decide on a signal your son can give you if he becomes overwhelmed and needs to escape for a few minutes.
Keep your brother's memory alive. Frame a favorite picture of your brother for your son to keep in his room, learn the special handshake and do it every night, etc. Have him write a letter to your brother and tie it to a balloon and watch it float away. Find reasons to bring up your brother- "Oh look, Uncle Ken would have loved that motorcycle", "Uncle Ken would have been so impressed by that drawing. Ken liked to draw too. Maybe that is where you got your talent", etc.
It is a very rough time for everyone involved and I am sorry you have to be going through this. Love, compassion and the support of friends and family will get you through it.
My prayers are with you and your family.