Cutting back on pellets is WAY more effective than cutting out greens. Pellets are calorie-dense. It is possible for a bun to get overweight from greens if you give really huge amounts, but a normal daily salad wont make anybunny fat. You can avoid giving root greens (carrot, parsnip, rutabaga, root celery, beetroot etc), stick with the leafy greens. Cutting back on greens (if he get lots of greens) can still be favorable if it make the bun eat more hay. Hay is very good for rabbits. It wears down the teeth (rabbits teeth must be worn down continously) and it keeps the tummy in good order.

I’d not cut pellets out completely. Cutting out all pellets can lead to too rapid weight loss which can cause life-threatening liver problems (hepatic lipidosis – liver toxicity). You want your bun to lose weight slowly and steadily. How much pellets is he getting now? You could perhaps start giving half of that amount. Make sure he has hay and water 24/7. Try to observe him so you see that he actually eats the hay.