Thanks for placing your query through HCM.
I can appreciate your concerns.
His leg pain & difficulty in walking is due to inflammation of muscles seen very often in flu
Taking pain killers & bed rest
Low platelet count
is also an expected finding in viral fevers like flu.
There are no reasons to worry.
He will be all right in next 3-4 days.
Hope I have answered your query.
Wish him speedy recovery.