A 16-year-old Aboriginal boy from a remote community presents to the local clinic with a two-week history of increasing fatigue, occasional shortness of breath on exertion, and migratory joint pains affecting his knees and ankles. He had a documented episode of acute rheumatic fever (ARF) three years ago, complicated by mild mitral regurgitation, and was commenced on monthly benzathine penicillin G injections for secondary prophylaxis. However, his adherence has been inconsistent over the past year due to difficulties accessing the clinic. On examination, he is afebrile. His pulse is 95 bpm, blood pressure 110/70 mmHg, and respiratory rate 18 breaths/min. Cardiac auscultation reveals a soft apical pansystolic murmur, unchanged from his last review six months ago. There is mild swelling and tenderness in his left ankle joint, but no erythema or warmth. His throat swab for *Streptococcus pyogenes* is negative. Laboratory tests show a CRP of 45 mg/L (normal <5), ESR 60 mm/hr (normal <15), and a normal full blood count. An ECG shows sinus rhythm with no PR interval prolongation. A point-of-care ultrasound shows mild mitral regurgitation. Considering the clinical presentation, history, and the significant burden of rheumatic heart disease in this population, which of the following is the most appropriate immediate management step regarding his secondary prophylaxis?