Replace meat in most meals with pulses or lentils. 100g of lentils is about 116 calories. The same amount of mince is 300 calories. Lentils make you feel fuller, digest more slowly so you feel full for longer, and incidentally contribute to healthy bowel habits. Almost every mince recipe can be made with one or other variety of lentils instead of meat, and you can make lentil burgers instead of the beef variety. If you have to use mince, use it half-and-half with lentils or add veg instead of half of the meat. For example, when I make spaghetti bolognese, I use very little meat but lots of chopped mushrooms and grated carrot.
Use skinned chicken or turkey instead of beef, pork or lamb when it has to be meat.
Change the balance a little on the plate: more pasta, rice or potato and less meat/lentil stuff. Never add butter to mash - use skimmed milk - and don’t use butter on new or boiled potatoes. Roast spuds should be a very rare treat, and chips too. If you do chips, oven chips are better than deep fried ones, but if you have an air-fryer, that’s even better.
Use smaller plates. They look full with less food on them.
If you do those things, you can cut the calories in every meal without an major change to the sorts of things you eat.
Do not keep biscuits, cakes, sweets, crisps or snacks in the house at all. If they are there, he will eat them. Keep fresh fruit available at all times.
Offer water or low calorie squashes, no fizzy drinks, not even sugar-free ones. Use semi- or skimmed milk for tea and coffee, and low calorie granulated sugar substitute.