It seems to depend on who you talk to, since I'm sure she will likely chime in, a certain breeder on here claims chins don't need any hay, I guess if your chin has perfect teeth and has very slow growing teeth (maybe by feeding a low quality pellet) pellets alone might be enough to wear them down. However as you pointed out, most people (owners, vets, at least several breeders) agree that hay is needed, which makes sense that an animal that evolved to eat mostly course grasses in the wild would still need to eat grasses (dried into hay) as a pet. Since they evolved to spend a lot of time chewing in order to get enough nutrients their teeth normally grow to deal with that. Chins simply don't chew nutrient rich pellets for long enough to wear down the teeth, most chins don't sit and grind up each pellet for a good minute or more like they do long stands of hay, it's normally a few chews and then swallow it.
It's more like 70%+ not 80%, or ideally a small handful worth of hay a day, the rest should be about 2tb (or about 30g since different spoons vary) worth of pellets a day. Sometimes reducing the pellets to just want an adult needs in a day (30g, if feeding a good pellet, contains enough nutrients for an adult chin, but do not limit if your chin is a kit) can encourage chins to eat hay since they have nothing else to eat after finishing the pellets. Even free fed most chins only eat about 30g of pellets a day in addition to hay. Keep an eye on his weight when doing that, if he is losing weight and not gaining back after a couple weeks then it clearly isn't working.
I know you said you already tried different hay, but have you tried hay like from specific small pet hay companies like Small Pets Select, Viking, or Farmer Dave's? Or just pet store hay? Another thing is the hay from a lot of areas in the US hasn't been great the past couple years, since most is harvested in the same area (Western North America) several brands are effected. I had been feeding Oxbow for example, but had to switch to a local hay because he started to refuse to eat it anymore, but happily eats the local hay. Also have you tried mixing types of hay, or even small amounts of chin safe herbs into the hay? Also, although it's not considered as good as loose long strand hay, have you tried hay cubes?
If you really tried everything and can't get him to eat hay at all, I would get him in for a dental check. It sometimes can happen that a tooth problem, like overgrown molars trapping the tongue, tooth spur, or a cracked tooth, can cause them to not want to eat hay but still chew on toys (which is done with the front teeth) as well as be able to eat pellets. If he checks out as there being nothing wrong with his teeth I would just let him be and get him in for yearly dental checks to make sure the teeth are staying in good shape.
