Have you been checking his gums regularly since he started this medication? Maybe it affects the pigment of the gums. They are nice and pink otherwise, so I would think that is good. (gorgeous white teeth too
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Watch him closely tonight and if he has any problems tomorrow morning, take him to a vet. I feel so bad for you that you have not found a doctor that you can rely on and trust. Have you tried looking for an AAHA accredited vet?
Sending hugs to you. Wish I could do more.