RHEUMATOLOGY-Help to lower 9yr. old's ASO level?

My 9 yr. old has had an ASO level of 425, brought down only to 335 after 5 months straight of penicillin. Any rheumatologist out there have any suggestions? He has been screened for rheumatic fever and cleared by a cardiologist. He occasionally has pain and weakness in his knees. Visually, they are not swollen. He has also been on daily Naprosin (sp?) since his diagnosis of juvenile arthritis last November.