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Hello and thank you for your question.
I'm sorry that your grandson is ill. Although lyme can cause joint pain/swelling and is a possible cause of his symptoms, usually the pain is not severe and not associated with fever. He was started on antibiotics (hopefully Amoxicillin for 3 weeks) because lyme tests are often very inaccurate and take several weeks to get results sometimes.
That said, the severe ankle pain your grandson is experiencing could be from a sprain sustained at the beach, and the fever could be just a co-incidental viral infection. A less common but more concerning reason for his symptoms could be a "septic" joint for which he would require immediate medical attention.
I would try giving him some ibuprofen and see how he feels an hour later. If the pain is unchanged and he still has fever, you should take him to the nearest emergency room. If, on the other hand, he seems comfortable after taking ibuprofen, you can probably wait until tomorrow to have him seen by his pediatrician.
Hello. I just wanted to follow up with you. I hope your grandson is now doing well.