Welcome to the forum, brilliant that you have come here to find the support you need for what will become a new way of eating. It is a very manageable condition with the right actions.
How much change will be needed depends on how far into the diabetic zone he is, an HbA1C over 47mmol/mol will have given a diagnosis but how much above that will indicate how much work he needs to do. He may have been given some medication like metformin if it is very high or been allowed to try by diet alone if not too bad. He also may have been offered statins for the high cholesterol and some blood pressure medication.
Learning as much as you can about the condition will help you make some sensible changes as it is often down to whoever does the shopping and cooking.
This link may help you find a way to modify his diet, it is a low carbohydrate approach which many find successful.
https://lowcarbfreshwell.co.uk/
Following those principals, I have reduced my HbA1C form 50mmol/mol to well into the normal range by diet alone and even though I have more fats my cholesterol is still good and the ratio excellent.
It is a good idea to make a food diary of everything he eats and drink estimating how much TOTAL carbohydrate (not just 'sugar') he has and reducing that gradually over a few weeks as that is less likely to cause issues with his eyes and nerves especially if he has a very high carb diet at the moment.
Low carb is suggested at less than 130g per day so you will be able to see how far off that he is.
I'm sure you will have plenty of questions but if you could post his HbA1C and any medications that will help people make suggestions. Some examples of current meals would help people suggest where saving on carbs can be made. It may be cutting some things out or reducing portion size and having extra protein and veg.