Hmm. What's he got to hide? Why shouldn't other people know why you can't come? If he's justified in banning you, he's justified in other people knowing about it.
His mother, his problem. If your elder child doesn't want to go, not a problem for you at all. He sounds ghastly Essbee, griefstricken or not.
I suspect that in ten years time or so, when feelings aren't so raw, he'll regret having banned you, and by extension, his mother's grandchildren, from her funeral. But that'll be his problem then as well, won't it?
Meanwhile, the idea of lighting a candle for her is a good one and maybe looking at photos of her and talking about her - having a "Grandma hour" (or 15 minutes - children's concentration span and all that) might help?