Hi, have you tried another dry kibble with lower fat% & a different protein with limited ingredients, also is his kibble a fish kibble??
My boy is 8yrs old & went off any kibbles that is fish/salmon, he seems to get acid reflux from the fish kibbles now, especially when they have fish or salmon oil, he grinds his teeth he has IBD, over the years I’ve learnt what all those weird things he does mean…..

As dogs & humans get older we don’t make as much hydrochloric acid in our stomachs, it’s called “Hypochlorhydria” low stomach acid, so it can be harder to digest food & food sits in stomach causing acid reflux…
I would not be feeding the Raw Hide, google, “how is Raw Hide is made”, Rodney Habib has a excellent video, it will shock you.
Can you cook a cooked diet for him, even if you buy tin tuna in spring water & add boiled sweet potato & veggies a couple days a week & the other days buy lean grounded pork or lean beef mince…. I make rissole, I add 1 whisked egg to 1kg (2lbs) lean pork mince or lean beef mince, finally cut up 1/3 to 1/2 a cup broccoli, 1 teaspoon finally cut up parsley, 1 grated carrot mix all together & make 1 cup size rissoles & bake in oven on baking tray 15 mins into baking take out drain water/fat & turn rissoles over put back in oven, I boil sweet potatoes & freeze the rissoles & sweet potatoes in sections…. When a dog stops eating something is wrong & its normally their stomach or pancreatitis… he’s telling you this kibble is no good & would prefer to go hungry then have his pain or acid reflux or have whatever he’s getting… If they could talk life would be so much easy, I would not want to be a vet, a lot of the times test results don’t show anything wrong, & its a guessing game until you work out a diet that agrees with him….