Hi and welcome to the forums.

I’m very sorry your rabbit is unwell. If a young rabbit drops weight very quickly, something is very wrong. It can be kidney disease for example. It is not possible to say without veterinary tests.

You can try supplementing his diet with rolled oats and a little bit of sunflower seeds. That would be safe, but start with small amounts so his tummy doesn’t get upset. It obviously won’t help with the underlying problem though. It’s very good that he’s still eating and playing.

When Wick was underweight, I supplemented his diet with Critical Care feeding, which is a powder you mix with water and feed it to the rabbit flr extra nutrients. I will highlight that Wick gradually became underweight. I didn’t really notice it until his vet told me, so his was not linked to a health issue as bam mentions. Are you noticing any behavior differences?

I suspect Wick became underweight because he was always on limited pellets that were Timothy , so he didn’t get a good start with being healthy weight by the time he grew into his full size. That being said, has your rabbit always been on limited pellets, including when young? I hope the solution may be feeding slightly more pellets each day and some rolled oats as bam says. Hay 24/7 should still make up the majority of his belly though.