That means you have few options: 1. Hire someone to stay with him in his home or get a family member to do it.
2. Get him to move in with a family member with consideration for what to do with his home.
3. Put him in either an assisted care facility or nursing home.
It doesn't sound like it is safe for him to be alone any more. The best option, if you can afford it and if he is cognizant enough to know where he is would be try to find a way where he can stay in his home and familiar surroundings, but that isn't always possible.
We've had to deal with this in my family and were able to care for the relative in her home.
Here is where it pays to have taken out an extended care policy on your insurance when you are younger. The rates have now skyrocketed for that, but part of the coverage is assisted care in your home.
1. his parents' conditions, physical & in the home...
2. talk to the dr. & get an assessment
3. get a hold on their financial arrangements and records.
4. find legal docs like Power of Attorneys for financial affairs and health care decisions. These should be "durable", i.e. ones that can be exercised when the person is incompetent.
These are best "in place" before any question of this competetence arises.
5. If your FIL is hospitalized, there should be a social worker at the hospital who can help you sort it all out.
This all takes time and lots of energy. I hope your husband gets some help not only from you, but the rest of the family.
Prayers....