Welcome to the forum
@Jogo1980
Sorry to hear about your Dad’s raised BG levels. Try not to be anxious - though they are really quite high, will probably be making him feel pretty lethargic and grim, with T2 there is often less immediate risk. Certainly wise to keep an eye on things, but hopefully he will have time to make some menu adjustments and/or try some additional medications.
The diet he is on may be fine for different people so I don’t believe the GP gave the ‘wrong’ advice, it’s just that it needs a little adjustment for your Dad. One of the tricky things about diabetes is there is no one simple dietary approach that works for everyone. Each person has a different tolerance to amounts and types of carbohydrate. By gradually reducing portion sizes of the main carbs in his meals, he may reach a point where his metabolism can just potter along happily. For others a more significant reduction in carbohydrate works better.
Keeping a food diary for a week or two, and making a reasonable estimate of the total carbohydrate content of meals and snacks can be really helpful. it can feel like a bit of a faff, and will involve some packet-squinting and weighing scales, but there is valuable learning in where the carbs are found in his meals, and what simple tweaks can lower the carb load.
As an example, creamy greek yoghurt with a spoonful of muesli and more berries, and dropping the banana, will be a very similar breakfast, but will have a much lower carb content than one which is mostly muesli. If the greek yoghurt is full fat it will help him feel fuller for longer.
Hope he manages to get the blood tests done without any hassle (given the current shortage of tubes!) and that you both get some reassuring support and advice from his GP.