It is great that you support your partner so well. If I were in your shoes I would write to him, it is hard not to read from start to finish when presented with a letter from your partner. You could focus partly on the facts and also the human side of you wanting the best for him. He will be aware of the consequences of poor control, so this will be tough, but if you can emphasise that you will help him, perhaps with diet, exercise and organising his control, hopefully he will come round. Very best of luck.