I am really concerned that your son is suffering from chest infection
has not subsided then probably he has not recovered completely. It is likely that the organism has responded partially to antibiotic. Its advisable to get evaluated by a good physician, if necessary do a chest xray, blood cultures and change to a better antibiotic.
Don't worry he will be alright. His infection has only partially responded which is clear from ESR
not coming down, fever not coming down. He needs further evaluation and treatment.
DR BAlaji C