There is NO way in the world in which this would ever be beneficial to even entertain this in any way, shape or form, including an annex or flat nearby. Of course your DH thinks it's a great idea, he's never home!
NFW!
No trail periods, no coordinating meals and care, blah blah blah, your DH does all that. ALL of it.
'FIL wants to move in with us'. 'That will never work in any way. I cannot and will not become a housekeeper or give up the freedom of my own home or my kids' for this. None at all. So you two discuss other arrangements'.
That's it. You make it very clear this is a non-starter and a total dealbreaker or you will wind up a housekeeper and carer for this man entirely. He will never change. He will expect the house to run on his terms.
My mother's seen many marriages end due to this in the 55 years she's been married, and always because the husband fundamentally believed his wife's role was to enable everyone else with no effect whatsoever on his own life bar the fun bits whilst he, the spouse, collected the money to go play golf. They all conveniently worked abroad or long hours and developed more and more excuses to not be home.
And I had plenty of friends from her friends whose lives were literally destroyed when Grandma or Granda was allowed to move in. It never worked well because the type of parent who does this is either already very disabled or very domineering.
Tempting as it is to be 'supportive', don't even start in any way enabling this: suggesting alternatives like flats nearby, annexes, trial periods; researching housekeepers, social activities food/feeding routines; looking for stories of how well this works (it doesn't), etc. Because this is all music to your DH's ears: whew, Gault will do all the work (on top of all her own) whilst I get that feel good factor.
And think of the impact on your kid and family. Nope!
'This won't work, DH. It's in no way good for your family life or marriage. You need to put aside this idea.'
Do not be bullied into this!