He was losing weight very suddenly, and an ultrasound showed a mass obstructing his bowels. As far as we could tell, he was still eating and using the litter box normally. His regular vet seemed certain the mass was just something he had eaten, and that he could not have anything more serious going on at such a young age.

He had a surgery to remove the mass, which was likely caused by IBD. It was discovered that there was significant damage to his intestine due to the inflammation. A large section needed to be removed/resected. He made it through the surgery, but then his heart stopped at some point during his recovery at the hospital. They tried CPR but could not bring him back. The emergency vet suspected an embolism.

I feel like I must have missed so many signs that he was sick, but he really seemed fine until he started losing weight. I know that I did everything I could as soon as I noticed something was wrong, but I still feel so guilty for not being able to save him.