Thanks for your responses, they are quite helpful.
Given what you have told me I recommend that you have him seen tonight at an emergency clinic if possible.
I don't like that he has been lethargic all day.
Vomiting once is not a big concern but what he vomited, which sounds like fresh blood, blood clots, stomach mucous and digested blood points toward very severe gastric irritation, possibly an ulcer.
His very pale gums and tongue point toward significant bleeding, probably in his gastrointestinal tract. I would be much less concerned if you told me his gums and tongue were a nice bubblegum pink. Blood and abdominal pain can make dogs very nauseous so I am not surprised he doesn't want to eat.
His lethargy may be due to abdominal pain, anemia and poor oxygenation due to bleeding or just that he is feeling very unwell.
It sounds like toxin exposure or garbage ingestion is unlikely so organ disease (kidney or liver disease, or severe pancreatitis) or a mass in the gastrointestinal tract leading to irritation and ulcers are possible causes of what you are seeing.
Whatever is going on needs to be addressed as soon as possible.
Please let me know if you have any further questions.