Picky eater losing weight! Any advice, supplements?

I have an older cat who has normally been a big eater. He still is always the one waiting by the bowl when I feed them (free feed wet food several times a day), but lately he will eat just a few bites and walk away. It's to the point he is noticeably losing weight.
He has been to the vet. Other than his megacolon which we have been treating for years, they have found no health issues. I have switched food. Tried everything from the most expensive raw food to cheap supermarket crap. Some he will not touch, others he eats a little and gone. I've even tried home cooked food.
Any ideas? Is there a supplement that might help his appetite, or help with general wellness if he's feeling off? He's older so could be having joint pain. Never very active or playful anyway so it's hard to tell.

Pain can affect appetite. Did they do xrays to see what the arthritis is? They can give him dasquin or adequan for glucosamine. They can prescribe pain meds tramadol or buprenorphine or gabepentin. How is his pooping? The pain meds may slow down digestion. They may give subfluids to keep him hydrated. They could give cerenia for vomiting and pain control. Or appetite stimulants if nothing unusual in blood work. When was the last blood work done?

He poops fine, I finally tweaked his megacolon meds to a combination that works. He hasn't been diagnosed with arthritis. Really I haven't suspected it until he started losing weight. Blood work done, nothing unusual. Had a dental in the fall and his teeth were fine. He acts hungry when I'm getting out the food, just won't eat more than a few bites. Maybe we need to go back for ultrasound or more tests.

I'm currently dealing with this with my older cat as well. Blood work was improved.she has early kidney disease. She would only sniff her food and bug me. I started hand feeding her kibble and she ate but not enough. She's on the heavy side and the vet worries about liver disease so I took her in Tuesday night. She always struggle with constipation so I have to give her canned pumpkin.

I wonder if your kitty caught a bit of a cold and can't smell her food? That was my second quess with my cat. So frustrating when they are fussy about eating n

Hm, I hope you figure out what's off with his appetite if there is indeed something wrong! In the meantime, there are a few tricks that make food more palatable for sick or picky cats:

-Warming up his food slightly can make it easier to smell

-Adding human chicken, turkey, or ham baby food (the Gerber or Beechnut in little jars don't have any onion or garlic added and are good for this) to his food might make it seem tastier

-Other things that you can sprinkle on top are bonito flakes, a little bit of canned tuna fish, grated parmesan cheese, some chopped up hard boiled egg, or Floriforta (it's a probiotic but the main ingredient is animal digest which is super tasty)

An update on my picky eater. Thanks for all the suggestions! However we have found the problem. His blood work a few months ago was normal, but yesterday tests showed his kidney numbers have skyrocketed. The toxins are so high it is affecting his appetite and he could even have stomach ulcers. We're doing IV fluid flush at home for a week, since I was afraid a hospital stay would stress him too much. Hopefully we can get the numbers down and start a maintenance program.
I have never had a cat live this long, so I really didn't know what to look for. Props to the vet for explaining it in plain English.