Hi, First of all has your puppy been kepted current on its shots? Is he lethargic or vomiting? Is he still eating? Anytime a puppy has diarrhea we get concerned because it could be number of things. When it has blood in it we always worry about parvo. However, sometimes when they have blood in their stool it can mean that they have a parasite. A couple that commonly cause bloody stool is coccidia or giardia. They are not covered by most routine dewormers so sometimes puppies will show up with it. I would recommend you get the puppy to a vet as soon as you can and have them run a stool check on him. They will be able to give him medicine if it is a parasite and they can help you rule out parvo. Even if he's been vaccinated against it they can still get it at this early age. It takes at least 3 boosters before they are fully protected against the disease and the last one needs to be when they are at least 16 weeks old.