First, you didn't mention his gum color, but you need to check that now. If they are pale, then the blood loss is severe and an emergency. Furthermore, even just the severe bloody diarrhea is a real worry. We can see this with Parvo and Distemper, but hopefully his vaccines will be protecting him from that. Still, hemorrhagic gastroenteritis, stomach ulcers, and some nasty bacteria (Campylobacter, Salmonella, E.coli, etc) can also cause these severe signs. And of course, toxins would be our other worry.
With this all in mind and the fact he is too nauseous to even keep water down (a red flag that he needs injectable anti-vomiting medication), it would be ideal to have him to his vet urgently. That way he can be treated for his nausea, started on IV fluids to avoid dehydration, and antibiotics to help get this settled without taking a toll on him. Though any delay in having him seen, you can at least try supportive care at home. To start, if he hasn’t just vomited (since otherwise we’d need to rest his stomach for a few hours first), then you can consider treating him with an antacid. Common OTC pet safe options would be Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) or Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac). Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do double check with your vet if he has a known health issues or is on any medications you haven't mentioned. Though again if he cannot keep it down due to nausea that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medication from his vet.
Once that has had time to absorb and he is steadier on his stomach, you can consider starting him on a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, cottage cheese, or scrambled eggs (made with water and not milk). The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset and reduce diarrhea. You can also add fiber (ie canned pumpkin, all bran) to this to bulk up stools. And it is worthwhile since we cannot use anti-diarrheals with those above concerns (since they can actually cause more issue if allowed to remain in the gut).
Since dehydration is a risk here, we need to keep an eye on his hydration. To check this and ensure he’s not becoming dehydrated, there are a few things you can test. Further to checking for gum moisture, you will want to make sure his eyes are not looking sunken and that he doesn’t have a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you are seeing any signs of dehydration already, then that would be our cue to have him seen before this becomes an additional issue for him (especially as it is often dehydration that makes them feel unwell).
Overall, there are a number of agents could trigger this for Jack. Since he sounds severely affected and the blood loss sounds profuse, this is an urgent situation that we'd want to have him seen for right away. Any delay and you can try the above until his vet is open. But once they see him, they can assess his hydration, rule out fever, make sure there is nothing in his stomach that shouldn't be there or any sinister viruses present. Depending on their findings, his vet can treat him with injectable anti-vomiting medication, fluids, gastroprotectants, +/- antibiotics to settle this for him.
Please take care,
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