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I am sorry your boy is having trouble.
Has he been licking his bottom more often then normal?
Is there any tarter build up on the teeth at the edge of the gum line?
Does he ever drag his bottom on the floor?
It is possible he is having trouble with his anal glands and licking the anal gland discharge is causing the nasty smell to his breath.
Over weight small dogs often have a hard time fully expressing the anal glands when they defecate.This results in excessive licking and scooting on the bottom in an attempt to empty these glands.
Here is more on anal gland problems in dogs
Often adding fiber to the diet helps firm the stool enough to express these glands. Added fiber will also help a lot with the weight loss.I would suggest giving 1 teaspoon of unflavored metamucil to one meal per day to see if this helps.
I would have the glands checked by the vet to be sure there is no infection, if these glands become impacted or infected they can rupture on the outside of the rectum and are extremely painful.If the glands are the problem you may need to have the vet express them every 5-6 weeks until he looses some weight
Anal gland expression is fairly inexpensive(less then twenty dollars) and can be done by a technician so it is normally a quick 5 minute trip.
I hope this has been helpful,please reply back if you have additional questions or concerns.
When a groomer expresses the anal glands they only do this externally this can actully create a the problem, The vet will express them internally. There are only two main reasons for scooting the bottom on the floor this is anal glands or parasites.
Because we see so much irritation from groomers expressing the glands we tell clients to ask that groomer no longer do this.
The fact that he is also licking more points to the anal glands. The is no other reason for the same smell to be coming from the butt and the mouth.
Please let me know what the vet says, I really suspect they are going to find that the glands are not being fully expressed