Agreed, whatever the relationship your husband has/had with his mother? He has to know now. What he does with it is HIS decision.
IF you don't tell? Or delay? More of those 'What IF's or IF Only's' will haunt you and him.
Not only are there the things that 'should' be done? There are likely other aspects of the whole shebang of things which will HAVE to be done. Estate wise and all that mess. Sooner aware? Sooner able to prepare and also cope. You do NOT want to be in the 'middle', just be there to support HIM and also HER as is needed. Of course, that's easier said than done, but it's always going to be THERE.
Also, the more family there is? The messier or easier it can get.
So get it out and get on with it! And know that others have had to do this, and they usually survive. Tho maybe with a little wear and tear.
's and be strong as you can -j