I’d have his teeth looked into, unless you have. A proper dental exam during full anesthesia plus dental x-rays. If a bun can’t chew properly it cant make proper use of the food it eats and it cant eat as much as it needs due to dental or jaw pain.
There are more sinister causes for drastic weight loss, and a full blood panel might give insights into what might be going on. A blood panel shows red blood cell count white blood cells, platelets, immunoglobulines and (other) inflammatory markers, blood glucose, liver enzymes, kidney function markers and more. Its a great diagnostic tool for rabbits.
Leg weakness is probably due to wasting (loss of muscle mass). It could also be neurological in origin, in which case E cuniculi should be suspected (e cuniculi is an intracellular parasite that commonly attacks the nervous system and/or the kidneys).
Rolled oats as ParsleyBun says are easy on the tummy, very tasty and can help the bun gain or at least keep weight on. You can also give a small amount of sunflower seeds and/or pumpkin seeds. Obviously start with really small amounts so as not to cause tummy upset.