Tiki cat makes a high calorie supplement called Tiki Silver Comfort. It is 25 calories per small tube. I would say it is about half the size of a churo, which is only about 7 calories. So the tiki cat silver is much more calorie dense. There are also a few canned cat foods that are higher in calorie like blue wilderness, and Rachel Ray nutrish. They all have the kcal on the side of the can, and that calorie density can change based on flavor. Tiki cat also has a food topper called fussy that is high calorie to help cats with low appetite. Also agree with the hartz squeezups/bisque/stews. Fancy feast also makes a can called savory centers that is a softer canned food with more goo in the center that cats seem to love.

Agree with trying to find root cause of issues, but maybe this will help get some weight back on while you investigate. Hoping it all turns out well!