This is quite interesting. Makes me wonder whether your Chi may be peeing in the kitchen, but you just don't find any drops because he walks around and steps on it or it gets absorbed if he sleeps on a doggy bed and by the morning the signs are gone versus you feeling the wetness when he sleeps in bed with you.
A good place to start is a vet visit just in case your dog may leak during sleep due to a weakened urinary sphincter. This form of urinary incontinence is a far more common condition in spayed female dogs, but occasionally may take place in neutered male dogs.
Alternatively, it could truly be your dog is marking when on the bed. I would expect him to get up and walk around when he does this rather than passively peeing during sleep.
The best product to clean messes is an enzyme-based cleaner such as Nature's Miracle. Some dog owners have had success with dog marking by making their dogs wear belly bands.
Last but not least, are you sure it's urine and not saliva? Some dogs may sleep with their mouths a bit open causing little puddles of drool.