I think understanding that anyone, what ever their age can be anxious is important. Children cannot and will not always be happy. The "i can see you are worried" phrase works well. Appreciate that you understand his stress. maybe ask if you can help? is there anything i can do to make you less sad? etc.
Can you offer a hug?
These children have been watching some really inappropriate messages on the TV about death and loss. Parents think they are not listening but they are and they are too young to understand fully. It may be time to have the talk between his friends about the virus and what has happened, but this is a call you need to make as I don't know if this will help or make things worse with this little one.
I had a young lad many years ago who's mum was expecting triplets....everyone went on about it for months but didn't explain it to him. He very definitely had depression. He would burst in to tears for no reason as soon as the babies were born and he knew what was happening he 'recovered'
Such a tricky time for all......