I'm sorry your DH is either being a twat or an ostrich :( Unfortunately many men are like this and it makes a very very difficult time even more difficult and worse still, brings yet another problem into the mix, where the other partner feels angry, upset, let down, unimportant and the relationship begins to break down as well - you need to talk to him so this doesn't happen :(
Christmas is a lot of pressure for most parents, for parents with a child as sick as Joseph it comes with a monumental amount of pressure. That need to make it absolutely perfect JutInCase :( I think the thing to hold onto is that it needs to be as perfect as you can make it for Joseph. So, forget all the time consuming, fancy, do dahs - Mummy, Daddy & DD there, Nan/Gran if she can be, any other favourite people, quick easy food that you know he will love, lots of boxes to uwrap with bright paper (can be a small cheap toy in each), lots of sweets (if he can have them) and most of all - time spent playing with Mummy & Daddy, pulling crackers, telling jokes and whatever else it is that you think will make his Christmas perfect, anything else anyone else wants they can sort out themselves. DD is too young to care :) I presume you have told all other adults that presents are off this year and delegated any shopping that needs to be done for other peoples children.... you need to take any other stress out of it.
Organise some nights out with friends and TELL DH he will be at the hospital with Joseph and book your Mum/babysitter for DD - have things to look forward to.
Joseph's results are looking good xxx