A 42-year-old man arrives for follow-up consultation. Three months ago he developed a proximal right leg deep venous thrombosis following a skiing-related fracture of the right tibia. Although not recommended by guidelines, a thrombophilia evaluation was performed, which revealed an elevated plasma homocysteine level, and subsequent genetic testing revealed that he is homozygous for the C677T methylene tetrahydrofolate reductase (MTHFR) polymorphism. He is otherwise healthy, and his medical history is unremarkable. He has a 12-year-old son. There is no family history of thrombotic disorders. His only medication is warfarin.

Which of the following is the most appropriate next step in management?