Does it start again awhile after you stop the drops and then clear up when your using them again and are they antibiotic drops? Did they put dye in to check the eye when you first went? It could be that theres something in it resetting up the infection maybe even an inturned eyelash. Sorry havent got chins but had a similar prob with one of my dogs once and it was an inturned eyelash that was setting up the infection every time the drops stopped.

I take it he is indoors? Im just wondering if he has a problem with tear production in that eye maybe its blocked and doesnt produce enough tears. Dogs and people with this problem get worse in dry air conditions such as central heating, air conditioning etc. So if it co-incides with cold weather and central heating coming on then maybe that could be it. If the eyes too dry and tears dont wash away debris they do become susceptible to infections. Also you said the sand bath irrates it? so that would figure too.

If its just a blocked tear duct I should think it could well be able to be cleared can be in humans and dogs. If its dry eye you can get drops that act as artificial tears and in dogs you use something called optimune that actually causes them to produce tears. Im sorry I dont actually know anything about chins. But if its just a problem with his eye I cant see a reason why the vets would say he needs to be put to sleep.

Hi im an owner of 13 chins (all rescues)

the sand bath should only be left in for a few minutes each day, it could be that your chin is slightly allergic to the sand/dust as one of my lot is, try cutting the sand bath right down to a few minutes every day or even every other day and see if that helps
also if its dust you use, or even fine sand, try changing to a chunkier sand
sand shouldn't be left in longer as they will pee and poo i it, then they end up bathing in the soiled sand