What Causes Persistent Cough In An Infant When Diagnosed With Croup?

My grandson is 6 months old. He has had fevers on and off for the last month or two and has been admitted to loma linda hospital for a week. He was diagnosed with croup and still had that cough even now but this fever will not leave him and has been to the ER dept twice and has seen his pcp twice. All we know it is fever of unknown orgin. Do you think he should have blood culture done? Has been breast fed since birth and my daughter limits people and kids who come near him or touch him. He also has a small rash on his torso but the pcp does not think its anything to worry about. She is very careful of new foods he starts on and will not try anything new without checking the ingrediants. He is very alert and cheerful except when his fever is up and according to his pcp his height and weight are good.