My name is***** you for your question. I am happy to help you. I'm sorry that your dog has an abscessed tooth. Those are very painful so I definitely understand wanting to relieve his pain. I will do everything I can to help you and ultimately him. I have a few questions to help me understand his situation better:
1.) How long has he had the abscess and how was it diagnosed? Do you plan to have the tooth removed?
2.) When did you give him the tylenol and what was the dose and how much dose your dog weight? What type of garlic did you apply to his tooth, how much and when?
3.) When did the diarrhea start?
4.) How is his appetite and energy level?
5.) Has there been any vomiting, or have you seen any stool that looks black or looks like tar?
Based on your description of what you have tried, it is possible that the Tylenol could be causing diarrhea. Tylenol is not a pain medication that is commonly recommended for dogs because it has a high incidence of side effects. Diarrhea, intestinal ulcers and vomiting are common. You said the diarrhea was reddish brown which could mean that there is blood in the stool. Feeding a bland diet of plain boiled chicken and rice can be good for a variety of causes of diarrhea. However, without knowing the cause, further treatment may be necessary. Please let me know your thoughts regarding the information I have provided.