Child Has Knee Joint Pain. Taken Ceporex And Ospen. Long-term Prevention Needed?

My son is 9 years. He plays gymnastics, got a pair of flat feet . He also has  bad tonsils (ping pong size) . He complained of right knee joint pain then few days later his left knee appeared more swollen and was painful simultaneously with the left knee but only on standing from sitting position. At no point were his knees red, hot nor tender. His ASoT was 800 , throat swab was positive for strept beta hemolytic group A( pyogen), ESR first hour 6 , Hb 14, no leucocytosis, no prolonged PR, normal echo and at no point was he feverish .  Currently he still arthralgia but it doesn t appear to respond well to burfen. He received ceporex for 6 days then switched to ospen 500 twice daily (wt 33). Please advise as regards diagnosis ( do u think this is post streptococcal arthritis) and if he is at risk of carditis and if he needs long term prevention and if Ospen is as good as LAP and what are the commonest side effects to expect. Thank you