I need to ask few more questions before answering your query. Early consultations can prevent complications. Kindly contact me on whatsapp chat  (84960595 six five) Dr Faisal Zoheb MBBS , MD PAEDIATRICS , PGPN (BOSTON), FELLOWSHIP IN NEONATOLOGY ( Newborn and vaccination specialist )

Greetings Looks like   benign acute childhood myositis (BACM), a temporary post-viral muscle inflammation triggered by the flu. The intense leg pain and tip-toe walking occur due to inflammation and tightening specifically in his calf muscles. The most effective way to help lower the CPK level and protect his kidneys is ensuring aggressive oral hydration with plenty of fluids. Keep your son on strict bed rest and avoid forcing him to stand or walk normally while the muscles recover.