I like yingyang�s answer.
I believe the child is getting plenty of religious education at school and does not need to be pushed any further by supplementing it with Sunday school. I would presume that the school also include going to church within their curriculum, but perhaps if you are that interested you may wish to take him along to a service on a Sunday, just to see what it is all about. Chances are, at 5 years old he will be bored rigid and won�t be in a hurry to go back.
I don�t think there is any harm in a child believing that their departed loved ones have gone to a special place. They will grow up and learn the harsh realities of life soon enough and then they can make their own decisions or discoveries. They rarely grow up to hate you for letting them believe in the nicities of life (God, Heaven, Santa, Tooth Fairy etc). Perhaps instead of going that extra yard for religion, you could involve him with some local groups which may or may not be connected to the church/school. Such things can supplement a child�s development and social integration marvellously providing they enjoy what they are doing.
Contrary to the view above, I think that children need to know about religion, including all the good and bad points and how the world has been and is affected by it.
From personal experience I don�t believe that this is brainwashing. I was raised with a strict catholic education and apart from having a pet rubber chicken called Fred and a shrine to the dearly departed Thierry Henry (from Arsenal, not Earth) in my garden, I would say I am a balanced individual. I am not a religious or societal nutter! And I don�t preach to others are bemoan and condemn their personal beliefs.