Snacks in a healthy balanced diet does not mean chips or lots of cookies or cakes etc..
He could eat diabetic friendly snacks, such as an apple and an ounce of cheese, or an ounce of nuts, or maybe a half of a ham sandwhich (or a whole one if his caloric requirements allow. There are some great articles here on spark that address this..... Remember that smart snacking is a good way to prevent over-hunger and keep the body metabolism up.
The other option would be, as someone else mentioned, increasing his portion sizes. Instead of 3 ounces of meat, he could have 4 ounces, instead of 1/4 cup mashed potatoes, he could have 1/2 cup, instead of a small salad, he could have a large one, and maybe an extra serving of the veggies. Just keep the ratio percentage the same and you will both do fine....