Thanks all of you. It really helps.
Joe is happy to be home and his sister is absolutely over-the-moon to have him here so she can rub his head. Her pure delight in the face of everything that is going on is keeping our spirits up.
Joseph is understandably tiring very easily - he has done more yesterday than he had managed in a whole week of hospital, sitting up and playing... DH had got him a playmobil castle and he and DD have spent hours playing with it - he obviously gets the knights to fight each other, she posts bits of jigsaws in the turrets 
We had a little trip out yesterday and then today we are off to the local stately home because they have some penguins there that Joe loves. He is very upset that he can't walk and I'm nervous that he will get upset when we have to walk past their play park, but we need to get out. We need him to have some fun and do some things he enjoys.
I guess I need to start a new thread... I'm not quite ready for life-limiting illness yet, I don't think, just in case the doctors tell us tomorrow it is all a huge mistake and the 5 or so consultants who have looked at the scans are all wrong. I just hope to god we have time to give him a bit of fun and happiness.
New thread will be in children's health still.
I have no idea how we can get through this.