Wait, didn't you post this before?

I'd take him to see a therapist. Yes, he could be very anxious about getting the virus, if he tends to be the anxious type anyway.

Have him do research on how the virus is transmitted. Being outside in moving air is way safer than being in an enclosed environment with others. Knowledge is power and since he is 13 and on the computer all the time like others his age, have him spend some of that time educating himself.  I share info and videos about the virus with my BD11 and she's doing ok thru all of this.  

I was just an indoor kid.  My poor stepfather, i remember him urging my mom to get me outside.  I wasn't afraid, just not a sporty person, liked to read and do inside things.  How aggravating it must have been to him, poor guy.

Is he addicted to gaming /screens?

YSD is getting anxious about this as well, she's 14.  She recently was asked by one of her (two) friends in the area where we live - within walking distance - to go ride bikes on the trails near our house. Great!  Said DH and I - it's a safe thing to do.  We all have been physically distant.  Bike riding would be great.  NOPE.  She's too scared, i.e. COVID and refuses to do it.  She's also refused to go to the store with DH (yes, we have masks) and refuses to be anywhere around other people.  We had our neighbors over for dinner (also, we know they are appropriately distanced) and she did not want them in our house.  Hey, I had to inform her of where I work (a medical facility also doing COVID research), what I learn every single week and how a very limited set of people are ok.  She ended up relaxing on this and had a good dinner with us. 

At one point she was talking about how everyone should just do nothing but stay home...DH and I pointed out she travels more than we do (back and forth BMs to DHs) and she has more exposure at BMs due to nature of the townhome situtation there.  I also told her if I cannot at least be careful and interact with the few people I do, and only interact with her and DH for the next year I would loose my mind.  She looked positively shocked for a few moments.  "blink, blink."

Risk and relative risk is a very difficult concept for even adults to understand let alone make daily calculations as to what are risks we are willing to take for ourselves and our families. 

I would get your ss to see a therapist if the thought of going outside is genuiely distressing for him and your attempts at using logic, facts and reason are failing to help. It is important that he is properly evaluated and helped rather than just being forced into distressing situations. 

My SS hated outside when he was young.  He would not go outside to play with kids or for anything.  So, he got to stay in the back yard.  He could go play with kids, or .... in the backyard by himself.  He would go to the back yard, sit on top of his play scape for two hours then come back in.  He had an uncanny internal clock and was always within 5mins of his two hours a day of outside "play" time.  

He probably came by his reticence to play outside fairly naturally.  There was a group of boys in the neighborhood who were his age give a take a couple of years.  They were all typical little boys, roughhousing, playing football, chase, etc, etc, etc.... SS was a crybaby and the boys all were pretty good about it except for one. The  youngest of them was about 2years younger than SS and was the little brother of one of the other boys who was my SS's age.  When SS would start crying the little guy would stomp, get frustrated and grumble about SS crying again.  Eventually SS quit playing with them. Rather than stopping the cry baby crap, he just self isolated.  we worked with him to quit being a cry baby when he played with the neighborhood boys. Nothing worked.  Unless an adult was there providing oversight, nothing worked.  When his mom  started back to college full time in the evenings she would drop him off at Kids Space drop in day care and I would pick him up 15-30mins later on my way home from work. He loved it.  There were usually at least a dozen kids there with 2-3 adults providing oversight, doing activities, etc... No rough housing allowed.  He just did not enjoy normal boy rough housing.