Start with a health check and a baseline
Before you change anything, book a vet check and get an accurate weight recorded. Rabbits gain weight for boring reasons — too many pellets, constant treats, too little room to move — but pain, dental problems and stiff joints also slow a rabbit down and make him eat differently, and those need finding first. Weigh him at home on a kitchen scale inside a carrier, and learn how he should feel: ribs easy to find under a thin layer of fat, a visible waist from above, and no folds of skin under the chin.
Hay does the heavy lifting
The single biggest change is making grass hay the diet again. Unlimited good hay or fresh grass should make up the great bulk of what your rabbit eats — it is bulky, fibrous and slow to chew, so it fills him up while moving his gut and wearing his teeth. Cut the pellets back to a small measured amount once a day and stop feeding muesli-style mixes altogether, because rabbits pick out the sweet, starchy pieces and leave the rest. Leafy greens such as herbs, spring greens and dandelion leaves can stay generous; the sugary items people treat as rabbit food — carrot, apple, banana and dried fruit mixes — shrink to rare, pea-sized extras or disappear.
Ration like you mean it
Weigh the day's pellets rather than trusting a scoop, put them in a treat ball or scatter them through hay so he works for them, and never let the bowl sit topped up all day. The most common reason careful owners fail is the rest of the household: someone else slips the rabbit a second helping because he looked hopeful. Decide the ration once, write it down where everyone can see it, and let nobody feed outside the plan.
Get him moving again
Food changes alone slim a rabbit slowly; movement does the rest. Give him as much secure exercise space as you can, for as many hours as you can, and make the space interesting — cardboard castles, tunnels, willow balls and hay stuffed into paper bags. Scatter his greens around the run instead of serving them in one place so he forages his way across it. Short daily play that encourages hopping and exploring beats occasional marathon sessions, and a rabbit with a bonded companion usually moves far more than a rabbit alone.
Track the trend, and respect the red line
Weigh weekly at the same time of day and write the numbers down; safe loss in a rabbit is slow and steady over weeks and months, not dramatic. The red line is appetite: a rabbit who stops eating entirely, passes far fewer droppings, or sits hunched and quiet is an emergency, because a rabbit's gut must keep moving — that means a vet the same day, whatever the diet plan says. At home, KinsLe's Care Teams keeps the slimming plan honest: the measured pellet feed, the greens and the weekly weigh-in become assigned duties with timestamps, so the ration cannot quietly double when two people both think the other one forgot.