To me visually, I'm not noticing anything looking angular or sharp and protruding in the hind end. But it can be hard to tell just by looks because of their fur. Feel along the spine and hips. If those feel sharp and protruding, he's likely underweight. If they feel pretty well fleshed and rounded, he's likely a good weight. But if you aren't sure, consult with your vet.
Maybe he was just dehydrated from not drinking much the day of and after the surgery, and has finally caught up in getting rehydrated. That would be my guess.
If he's taking longer to finish his pellets, it can mean his growth is slowing, so he's not feeling as ravenously hungry and so the pellets aren't getting eaten as quickly. If they're there at the end of the day, then he probably has plenty of pellets, if he is also maintaining a healthy weight. Or taking longer to eat his pellets may just mean he's liking his hay more, so is eating more of that during the day instead. You will need to check and make sure his hay intake hasn't decreased, or that could be why he's losing weight, if you find he actually is underweight.
If it's that he's eating the same amount of hay as usual (or more), but is getting skinnier and not staying at a healthy weight, the issue could be with the cut of hay and the protein level. Protein levels in hay will vary depending on the variety, the growth stage it was cut, and if it was dried properly (over sun cured yellow hay, or very old hay, will have less nutrients preserved).
https://wabbitwiki.com/wiki/Hay
So a more mature coarse hay with mostly hard stems, is going to be the lowest in protein and nutrients, which adequate levels are needed to support weight maintenance and health. But a coarse hay is higher in indigestible fiber which is good for digestive motility, but can be bad if too much is consumed and not enough nutrient dense food. This type of hay can be good for rabbits prone to slow motility issues, but may need increased pellet amounts fed to provide the needed protein and nutrients to get the right balance for weight maintenance and needed nutrients for good health.
A softer hay with lots of green leafy strands and less hard stems, will be a much richer hay in nutrients and higher in protein, but much lower in indigestible fiber. This type of hay can be good for growing rabbits, but may not provide enough indigestible fiber for good gut motility. And in mature rabbits, can be too rich of a hay to feed exclusively.
So usually a second cut hay that's a good mix of both leafy strands and hard stems, is the most common one fed to pet rabbits, and is most often the best cut to feed as it has the best balance of nutrients and fiber. With this type of hay being fed, the right amount of pellets isn't usually as critical, as lots of nutrients are coming from the hay. I've had rabbits with digestive sensitivities to pellets, that were exclusively on a pellet free diet and unlimited second cut grass hay, that very easily maintained a healthy weight and good over all health. So low pellet intake doesn't necessarily lead to weight loss if the hay has the right balance of protein and nutrients, and the rabbit is also getting fed some fresh veggies and/or forage.
If you have a coarse hay, but need more protein and nutrients in the diet because your rabbit isn't maintaining a healthy weight, either a softer cut hay will need to also be fed, fresh grass/forage, or more pellets, to provide the needed protein and nutrients.
Or there's the opposite where a rabbit is getting too much nutrients and is getting overweight, or producing excess uneaten cecotropes from the excess nutrients being processed in the cecum, then less pellets and more fiber is usually needed to get the right balance of protein, nutrients, and fiber.
So it all has to do with the right balance of protein, nutrients, and fiber, in the hay most importantly and/or forage, and then some in the pellets and fresh veg, to provide what each individual rabbit needs to maintain a healthy weight and overall good health. It's kind of a complicated thing for new rabbit owners to have to try and figure out. Which is why the recommendation to feed limited pellets and unlimited hay, isn't necessarily the best advise without a more detailed explanation of the finer points of hay and pellet composition, and balancing the diet.
I have yet to find an online source that adequately covers these finer points, so I can't direct you to any one resource to read about this. But here are a few that cover some aspects of diet and how a rabbits digestive system functions.
Rabbits digestive system
https://www.merckvetmanual.com/exotic-and-laboratory-animals/rabbits/nutrition-of-rabbits
https://www.harcourt-brown.co.uk/articles/free-food-for-rabbits
if anyone knows why he could be doing this I would really appreciate the help!! Thank you!
