Why my canary has no feathers on his face?

Feathers can take 6 months to a year to grow back. If the damage was severe enough it could take longer. Also, there are numerous bacterial and parasitic diseases that could be affecting him. You could take him to the vet, but I think his feathers will probably grow back in time. Just keep a look out for any other symptoms like thickening of the tissues of the cere and/or legs, dirty face, watery/swollen eyes, poor feather condition on body, etc.