Hi Emma and welcome to our forum.
First thing you need to know is that there is no such thing as "diabetic food". You will see in chemists and supermarkets cereal bars, chocolates, biscuits, etc labelled as "diabetic". What this means is that instead of making them with sugar they have used a sugar substitute. The general advice (for example from Diabetes UK) is don't touch them. They actually don't taste very nice and the substitute they use is a pretty powerful laxative. So if you ever buy your dad a bar of "diabetic" chocolate, just make sure you don't get between him and the bathroom door!!
Basically a diabetic should eat the same as a non-diabetic person, but just needs to eat less of the things that cause a large increase in blood sugar, These are obviously sugar itself, but also the starchy foods - bread, pasta, rice and potatoes. That doesn't mean your dad isn't allowed to eat these - but he needs to eat smaller portions of them than he is probably used to having. So, for example, have only half the potatoes than normal but more of the other vegetables, or a bigger portion of meat.
A diabetic may have blood sugar that is too high (hyperglycemia) or too low (hypoglycemia). With high sugar levels you might feel listless and sleepy, but not usually grumpy (unless of course he is just angry at having diabetes). One of the commom symptoms of low blood sugar is becoming argumentative, irritable and impatient (but then some people are like that anyway!).
Why not get your dad to log in to the forum and have a browse around. If he (or you) have any questions or need some advice, there's always someone here who can help you.