You can make a balloon or streamer wall for him to run through. Buy a plain cake or cupcakes and he can help decorate. Have a special pajama day. Have a birthday picnic. Gather family & friends on FaceTime, Zoom, whatever video app for singing. Piñatas are still fun even if it’s just your household. Buy a sprinkler and have a water day.
I’m planning on a piñata, cake, presents. Maybe get a little car parade together. Put a sign in the yard saying honk for my birthday. That’s all I’ve come up with.