What Are The Implications Of High ESR Level. My 11

What are the implications of High ESR Level. My 11 year old son had mild Viral Fever between 12th and 14 December and was treated with Paracetemol and Azythromycin. However the fever returned on 18 Dec and shot up to 104 degrees on 19th Dec. In a blood test for MP and CBC on 20 Dec, other counts were normal but ESR level was 50. Fever came down on 21 morning to 98.1 to 98.8 levels, still on 450mg paracetemol dosage 4 times a day and Azythromycin once a day. What is the implication of high ESR level here. Does one need further tests / consultation on this issue, or this is normal in case of high fever and subsides as fever comes down. Regards.