Thank you for your query.
I can understand your concern. However you have not mentioned from when is the red line present and whether there is any pain. Such a line is not due to blood poisoning
but any inflammation at the knee can give rise to inflammed lymphatics which may appear as red lines. There should be some pain around the area. I suggest you do hot fomentation and observe. If there is no improvement then consulting your doctor would be appropriate.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.