He cannot change his genes. Whether he normalizes his blood sugars or not, he will still be diabetic.
A thorough annual exam, including neck exam is recommended for everyone, with or without a family history of thyroid cancer. Self-examinations are explained in pamphlets offered by the American Association of Clinical Endocrinologists.
Thyroid cancer may develop in patients with a normal, over and under functioning thyroid gland. There is no routine lab that can screen for thyroid cancer. If a neck lump is felt, your healthcare provider should order a neck ultrasound. There are no vitamins, herbs, or supplements that can lower your risk of thyroid cancer.
Enjoy an active and healthy lifestyle. Know that most patients with thyroid cancer do very well. Keep a close professional relationship with your doctor(s), ask them questions, and follow their advice.