Water. At least 2 litres a day, really.
And that artificial sweetener acts as a laxative!
Increase vegetables such as cabbage, cauliflower, broccoli, leafy green vegetables generally. They are all low in carb.
But I find water is the crucial think, it keeps the stools soft and easier to pass.
If he is getting bleeding, you may want to discuss with a pharmacist. It is very painful, and there may be something that can help that area heal.