Parvo is one possibile cause for the symptoms you describe. If your dog has gone through his entire puppy vaccination series (there would have been 2 - 4 vaccines between the ages of 6 weeks to 4 months), then he should be pretty well protected against Parvo, but if he did not complete the vaccine series, then he will still be susceptible, making Parvo a more likely possibility.
The symptoms you describe are pretty general, so there are many things that could cause them. These include several other infectious diseases besides Parvo (bacterial or viral). Also, being a puppy we always worry that they having eaten something he shouldn't - this would include poisons, getting into garbage, or foreign objects like rocks or toys. These dogs will often have some vomiting as well. Metabolic diseases such as kidney or liver problems are usually seen in older dogs, but these are also possibilities.
Without knowing exactly what is going on, I cannot tell you if whatever is causing your dog's illness could also affect your cat, but it is unlikely. Most viral infections do not transfer between species.
Because the symptoms have been going on for several days and it sounds like they are getting worse, I would recommend having him seen by your vet as soon as possible. Hopefully, they will be able to get him started on the road to recovery very soon.
I hope this answers your question as to what could be causing your dog's illness. If so, please click Accept. If you have additional questions, please let me know.
