I mostly use Oxbow, but when it comes down to it hay is hay regardless of brand. Over the years I've used all kinds of brands like, kaytee, ZuPreem, American Pet Diner, as well as local farm hay. The kind your chin will eat is the best for your chin, make sure the hay looks and smells good, chins wont eat it if it's stale, dusty, or moldy. It's best to have a variety of hay though, although timothy is the most balanced, other hay types and cuts of hay add flavors and textures. You can also try mixing hay types together, adding a small amount of alfalfa hay into the timothy for example might make it tastier.
Fuzzies Kingdom even sells foraging hay which chins seem to love, it's often sold out quickly, but they also sell just the hay topper that you put in your existing hay too.
You can also try hay cubes, some chins seem to enjoy the different texture. Personally I feed my chins 2-3 different types of hay at a time, along with alfalfa/hay cubes. Once a month (because they are expensive) I give them a Oxbow timothy hay stack puck.
Another thing you can try is giving smaller amounts of hay, rather then giving the whole days worth all in one go, give some in the morning, then some later in the day. Some chins don't seem to understand that the hay is not unlimited and it's for eating not playing. Most chins wont eat hay once it's on the ground, and some prefer a bowl of hay over a hay holder on the side of the cage or hanging.
Eating hay is very important, and should actually be the primary food 70-75% of the chin's diet. Not only does the hay provide fiber it's also how the chin wears down their molars, and different types of hay wear down teeth differently. Chew toys only wear down the incisors and pellets are too soft to wear down the molars.
You said the chin also has dental disease? What kind and what is the treatment you are doing? Are x-rays taken to make sure no root involvement? If the teeth aren't worn down and become overgrown they can cause the roots to grow up into the skull and down into the jaw. Overgrown teeth can also curl over the tongue and trap it. If it hurts to chew then eating hay is not going to be something he is going to want to do, no matter how tasty.