Thank you for your query.
Weakness in legs following fever
needs to be taken seriously. I suggest that you consult your doctor who shall examine the child and ascertain a cause for it. He will have to do a complete neurological examination to try to find a cause for it as there are many causes for such weakness especially after a head injury
following a fall and also the vaccination
I hope I have answered your query to your satisfaction. Feel free to revert back in case your require any clarification.
Wishing your son all the best....