Diabetes mellitus, or more commonly known as just diabetes (both type 1 and type 2) raises blood glucose level so you must not feed him anything that contains sugar.
Type 1 diabetes results from the lack of insulin due to the primary faliure of insulin-producing liver cells (insulin helps take up glucose into tissues, thereby reducing its level in the blood). This means he needs treatment that injects insulin into his circulation. All you can do to help is watch out if his diet is sugar-rich and then tell him off. lol
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Primum non nocere
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