What's this?

On saturday my son had a bite on the back of his neck and woke up the next morning bright red (i'll add a picture below) he had calpol and piriton. Monday he was better the rash had gone but still had a temperature. Yesterday he was off his food he would only drink which is unlike him cause he loves his food. This morning he woke up this all over his tummy (picture below) its itching him i just dont know what it could be