My 5 year old woke up this morning with extremely sore calf muscles and is unable to put any weight on them - so unable to walk. He has been unwell recently with a temp but seemed to be fully recovered yesterday and feels fine otherwise.

This happens. This is a less known but common finding in influenza B infections or in any flu like illness that moves into that muscle group. If it is a simple flu, it would gradually improve over days/weeks. It can be a localized muscle inflammation from another agent & a review of the kids history/exam and testing is needed to sort out this issue.