what to do for an 8 year olds birthday during a pandemic?

Bake a cake and give him gifts.  No reason why people have to think their kids must have a birthday party every single year.  Heck - the only birthday party I had was when I turned age 16 and the same was true for my classmates.  Most didn't have any birthday parties.

All the time I was growing up, the birthday person got to pick the entire dinner meal.  We had birthday cake/candles and ice cream for dessert.

You and your kid will survive not having a big birthday party.

Just make him a cake and buy him a gift.

Make a delicious beautiful homemade cake for him/her. Homemade food is always better in every aspect. Tastier, safer, cleaner, and healthier. Or get it home delivered by a reputable bakery - it should be fine. Covid19 has been apparently been scientifically proven to mostly only be transmitted through the air between people. It seems that it's not really a food-borne illness like the ones caused by Salmonella or Staphylococcus aureus. A homemade cake would be much better, of course.

Bake a cake for him or let him join you in preparing his favorite dishes. Just spend and appreciate his simple birthday together. Explain to him the situation why you cannot celebrate his birthday with friends/family. I'm sure he will understand.
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I would do a party per say with guest.

We use to live in the county when our kids were small. On their bd we just celebrated them all day.

It started with their favorite breakfast, and a few treats.
Then their favorite games.
Plus a few treats.
Their favorite lunch, dinner and snacks.
A swim in the pool, water balloons squirt toys
Cake and ice cream and the grand fanale was their own pinata hung in a tree all for them. They could hit swing anyway the wanted and they could decide to use blindfold or not.

These days you could do all the above and have friends, family neighbors drive by, cards signs, balloons all put on the yard

My neighbors had a "drive by" birthday party. The child and parents sat under the carport while family and friends drove by their house, picked up a piece of cake at the end of the driveway (it was put on a table so there was no person to person contact) and left a present. The carport was decorated with balloons. 

Just give your kiddo a simple yet a memorable birthday celebration. It would be nice if you will have that theme that focuses on his/her passion. What does your kid loves to do? If he/she loves playing lego, then it will be amazing if you'll have a lego party. Make some arts together. Have that mother-daughter/son enjoyable moment. Surely your child will be happy.

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Take the day off of work to spend time playing with your kid.

Give it a sack of coal.

Have him invite all of his friends to a video conference birthday party.  Tell all of them to ask their parents to prepare a favorite meal for their child, planning on eating together at the video conference, and he can open his presents on the video conference too.  Choose some games where they don't have to be together, like guessing games, geography, etc.  Speak to the parents and let them know what you are doing and ask if they can have their kid ready for a Zoom conference.  If they need help downloading the app, offer the help.  It is easy to do, very user friendly.  If you haven't used it, I suggest a trial run a couple of days before.