Weight gain tips/tricks for very picky boy

Waylon is almost 15, and was diagnosed earlier this year. Don't really know what stage he is as my vet doesn't put it in those terms.

He's been 18lbs most of his life and he's down around 14 now.

He refuses to eat any kidney food, so we've been giving him various wet food. We're also giving IV fluids every other day and that has made a big difference in his energy and demeanor. I have mirtazepine and zofran as well.

He is very picky and won't touch any of the calorie supplements I've tried like the high calorie gel, tiki senior, prescription a/d, boiled chicken, etc.

Any other ideas to help him put on more weight?