Rabbit not eating cecals and feeling skinny.

Hi, I've recently got a new bunny. Carried on his diet as it was at the rescue but then he started getting a little bit of runny poo so I cut back on veg, only giving a small bit of spring greens on a night.

For the past couple of days I've found a lot of cecals uneaten and he feels really skinny.. Can feel his hip bones.. I'm really worried he's losing weight and I don't know why

He's completly fine in himself, hopping around, munches hay and pellets like there's no tomorrow. Any ideas?

I would take him to a good rabbit vet and get him checked out if he's losing weight