My vet says my Labrador is "a bit heavy", but I don't overfeed him and I'm not going to starve him

Start by finding out what he actually eats — not what you think he eats. Weight is the single biggest lever you have over how long he lives and how well his joints hold up: dogs kept lean live meaningfully longer and need less arthritis treatment. So this is worth doing properly, and slowly, and nobody is asking you to starve him.

Two concrete jobs this week. First, stop using a cup and weigh his food on a kitchen scale — a cup varies by tens of grams depending on how you scoop it, and that difference alone is often most of the problem. Second, write down every treat, chew, dental stick and scrap of toast from every person in the house for seven days, your father-in-law included. That list is where the weight almost always is. Treats should stay a small minority of his daily calories, and a lot of them can be swapped for a portion of his own kibble handed out by hand — most Labradors are just as pleased with that.

Then track it. Weigh him monthly at the same time and place, and learn to feel his ribs: you should feel them under a thin layer, like the back of your hand, without pressing hard. Ask your vet to check the medical causes of weight gain — thyroid, some medications — before anyone blames willpower, and to set a target weight and a safe rate of loss with you. Crash diets are genuinely dangerous; in cats, rapid weight loss can cause liver failure, so the rate matters as much as the plan.

Build his exercise up gradually rather than all at once, and be careful in the heat: an unfit, heavy dog overheats easily.