I can't answer your question from a medical point of view, and obvously I have no idea of the severity of your son's condition, but on the face of it I think I would allow him to attend. In my experience children are far more matter of fact and accepting than we adults - which is why they don't 'get' the sadness of it all, and take funerals in their stride.
To give you an example, at a friend's mother's funeral, she sat next to her little granddaughter. Obviously my friend shed a few tears, and when the little girl saw her crying she said 'What's the matter grandma? Don't you like the music?' That's kids for you!! :o)
I'm so sorry for your loss. I know what a difficult time it is. Much love. x