Perfectly normal!
There are so many levels to the grief, and it can be very overwhelming.
Those discussions about what he wants are so important, but really difficult to talk about and pushing it is hard. Maybe once he gets there to live with you it will be easier to find some ways to talk about it maybe with a health professional starting the conversation up for you.
My Chris wouldn't talk about much of that either and usually it worked best to have someone else start the conversations. Not always, but sometimes it did work.
We can't predict how long anyone with MND has, and on this forum we see all the differences. My Chris only got 11 months from diagnosis, though he had struggled for around 6 months before that. Some have been far faster, while some get some really good years.
We learn to try to just do one day at a time, as today is all any of us have in truth. It's not easy, the big picture does grab us all at times, but it helps a little.
I'm very glad you have reached out for some support. I will just mention I met Sam on fb and whipped her over here before she could blink