How To Reduce High Anti-streptolysin O Antibody Values?

My son is taking pentid 800 mg twice a day but his ASO TITRE is not coming down below 200. it has reduced from 500 to 300 in last 3.5 years during which he had taken medicine. His age is 15 years. what should I do? He was detected rheumatic arthritis in Aug 2010.