How do you weigh social distancing against isolation when making decisions about a loved one who lives alone?

How about meeting him on the patio or driveway with some lawn chairs to have a visit? Keeping your distance could still be accomplished and you'd be able to see how he is doing.
If he can use a computer or cell phone, you could have video chats with him.
If these things won't work and he understands the seriousness of covid19, ask his doctor if a home health person could go to check on him, things to be done at the house. It will be hard for you to get your hands on protective coverings, but if you can - maybe you can work out a visit wearing the right clothing. This is very hard as a caregiver in the home - you have a really tough situation.

HE IS 93 YEARS OLD...... Don't get me wrong... but he has a good life.. DO NOT FEEL GUILTY IF HE GETS THIS DISEASE... You also pointed out he has a familly of 10 AND EVERYONE HAS PITCHED IN........... GUESS WHAT? if he GETS THIS VIRUS... YOU ARE NOT THE ONLY ONE TO BLAME.. EACH OF YOU TAKES HIM TO YOUR HOME... SO DON'T THINK YOU ARE GETTING OFF EASILY BY SAYING YOU ARE GOING TO FEEL GUILTY... Make him dinner and know that it is one of many dinners that MAY OR MAY NOT GET HIM THIS VIRUS...I am sure your meals are not deadly... or your family's...
MAKE HIMI DINNER, AND DESERT AND KNOW THAT YOU AND YOUR FAMILY ARE NOT TO BLAME IF HE KEELS OVER TOMORROW MORNING... It is NOT YOUR FAULT...
If he gets lonely and starves to death, then YES, BLAME YOURSELF...
Life is going to happen. with or without your meals.

Do Not change your routine.... HE WOULD BE SUFFER MORE IF HE DIDN'T COME OVER...
I heard on the news today,,, 1,400 people died over this weekend in Italy? Did I hear that correctly... The caskets were piling up faster than they could lay them in the ground... Crematoriums are running non-stop in Italy, and IN CHINA..

I heard that China cremating so much non stop to try to curb this virus..
I don't honestly know... But I do Know That your FIL needs his family more now no matter the cost... HE WOULD RATHER BE WITH FAMILY THAN ALONE.... Give that to him..

I agree with the answers below that say to VISIT HIM.
Loneliness is way worse for him from lack of human connection.

I don't know what to do about my mother. She is 85, has had two bouts of lung clots with pneumonia and lives in AL. She won't use any form of technology to keep in touch, even finding the phone too much for her usually. The care staff seem to be meeting the basic needs of the residents, but we are of course worried about the long-term results, as she has narcissistic personality traits and is probably depressed already, as she regrets her move to live near us even though we have done/provided almost everything she has needed since she came. My mum is not at all sociable and her heavy dependence on us, often accompanied by complaints and criticism, has really got us down, so although it seems awful to say it, not having to go round there so often is a respite for us. I suppose we could walk past her apartment (top floor, annoyingly)and wave, but it doesn't seem adequate. I suggested asking if she would like an iPad to keep in touch with the family, but my brother laughed the idea away.

My opinion is that you should choose one person to be allowed to visit. Then he won't be lonely and less chance of getting infected. My dad is almost 96. I was getting all worked up about him getting the virus and then I told myself, he lived through WW2 in the trenches, was wounded by shrapnel and has lived with PTSD ever since. He is healthier than most and he sleeps alot. He will survive.

Dad and I are taking it day by day. He is 91 and lives alone. With everyone having nowhere to go, people are calling him a lot more! I generally stay with him Wednesday and Sunday nights. We decided I would skip today's visit, as he has plenty of food and is in good spirits. Ultimately, I let it be his call. If he starts sounding depressed or isolated, I will go over. I am trying to limit contact from other people.

It's very hard because I have spent the past year preventing social isolation.

Today is March 31, 2020... We shall see what April brings us. On NPR radio, this man set up a "ring.com" for his grandpa, weeks before covid19, just to keep tabs on him to make sure he is safe when he can't get there. Anyway, he found this to be a cool way for safe distancing. He rings his grandpa every morning when he walks out to get his newspaper. Grandpa thinks its strange talking to a doorbell, (kinda sounds like an episode from Get Smart) but he is getting used to talking to a doorbell, as he enjoys being able to talk to his grandson.. :)

Great podcast specifically addressing this from medical POV : https://www.washingtonpost.com/podcasts/post-reports/feeling-lonely/
if the link gets cut it’s the Washington post April 3rd post reports- and there is a transcript you can read but it’s far better to listen and hear the emotion in these people’s voices...