Thanks for your response! As it turned out I started to see blood in Fred's urine not too long after I took these photos, so I took him to the vet immediately and it turns out that he has a bladder infection and was swollen with water. All that extra size wasn't weight from food at all. He's looking better now after a couple days on antibiotics and a lot of the swelling has gone down, though he's still puffy towards his back end. Hoping for a full recovery for him soon. I hope no one else has gone through anything like this, it's been really stressful and I was terrified I might lose him. Thankfully he's been a brave boy and is looking happier already.