A 5-year-old boy from rural Maharashtra presents with progressive joint pain, stiffness in the knees and ankles, and dark brown discolouration of the sclera and ear cartilage. His urine darkens on standing for several hours. Serum creatinine is normal. Urine chromatography shows elevated homogentisic acid. Genetic testing confirms homozygous mutations in the HGD gene. What is the most appropriate next step in management?