Hmm...well I'm always for treating holistically and naturally rather than synthetic meds and my girl is older and fussy too but here's what I would do
-Stop free-feeding him and offer him breakfast and dinner (you'll see they get muuuch less fussy when they've had a bit of time to consider it and get hungry) this has helped me with Jacko a TON.
-At this point, the important thing is to offer him veggies in a way that he will eat and to sneak in the less tasty stuff. For Jacko this meant offering it all blended and mashed together into a weird sort of loaf/pie with whole egg and plenty of banana mush and orange juice to make it sweet and mask the yucky stuff. The result? She's gobbling sweet potato, broccoli, kale, arugula, bok choy and all the good stuff and she doesn't even notice it. If your little guy likes eggs etc use it to your advantage.
-As far as the silymarin (which is the active compound in milk thistle) I would not give it to him long term just in case. I learned via Shauna Roberts over on FeedingFeathers that giving it long-term is thought to cause the liver values to become abnormal over time according to some herbalists.
-Diet supplement-wise I would give him plenty of dandelion and alfalfa (good purifiers especially for the liver with dandelion) and add in some probiotics and some chamomile tea or cinnamon (anti-yeast). Personally I'm against feeding artificial diets like pellets and the like but thats up to you. I believe we can accomplish a lot of healing through food.
-I'd scrap anything being put in his water due to the fact that many products in water either dissolve and lose their potency and/or foul the water making it dirty and becoming an infection risk.
I think if you tailor his diet to be more low-carb-ish (to watch for yeast) and full of greens he'll perk up---he'll be stubborn sure, but any animal can learn to eat well given the time

Good luck
