You should not freak out but just be mindful that it is a very serious disease. My dog was an absolute princess, spoiled rotten, from a very reputable breeder, champion sired - all nine yards. Yet, she had it in one day and died in three. Her diagnosis was an acute aspiration pneumonia, which means that it started out of nowhere. After doing a fair amount of research, I realized that an aspiration pneumonia is usually related to chocking on food or other substance. I wonder if your x-ray showed a presence of any foreign substance in his stomach that he could have chocked on.

When bacteria enters the lungs, the inflamation of the lungs causes pneumonia. The antibiotics should help your dog to get rid of this bacteria. I am surprised that your vet has not done blood work and has not checked upon your dog's oxygenation level. The increase in the white blood cell count signals a presence of infection in the lungs. The count also signals the severity of the infection and whether or not it is under control. How does your doctor know that the antibiotics are helping your dog? He needs to measure his white blood cell count against what it was before. Ideally, antibiotics have to bring it back to normal. Since pneumonia causes the swelling of lungs due to the infection, it affects the amount of oxygent that your dog's blood receives. That' s why he had to check his oxygenation level to make sure that he is getting enough oxygen. Otherwise, the dog should be put on oxygen if it is not enough.

Also, I would not recommend you to play very physically with your dog since it affects his breathing.

Are you sure that your doctor diagnosed your dog with pneumonia? It sounds like he treats him more for bronchitis. If he did diagnose him with pneumonia, did he say what kind (there are multiple kinds and there are multiple corresponding antibiotics) and did he say whether it affected one or two lungs?

aspiration pneumonia
 

Aspiration pneumonia is the worst kind of pneumonia to recover from and many dogs die from it within days. If his pneumonia was diagnosed soon and he is on antibiotics he will probably be fine in a few days. He should get extra rest as some other stated here though. It doesn't sound like he had aspiration pneumonia

yorkie pneumonia
 

my Mia was perfectly fine 2 days ago. In 1 day she started a cough...threw up...trouble breathing. Called vet..they saw her next morning and told me she would most likely die. SHOCKING. They gave her a shot of antibiotic and a can of protein food supplement and recomended the vet hospital. I was reluctant to go there...extremely expensive...but after watching her struggle to breath, went. They put her on oxygen and pretty much said the same thing...I could keep her there. Approx bill over $5,000 and 50/50 chance she could pull through. She is only a 3lb., 4 month old.

We took her home...is on 2 different antibiotics. Are giving her a syringe of water/pedialite a bit of nutracal and protien food suppliment about every 1 to 2 hours. Have also put her in a steam tent (boiling pot of water and towel) for 10 minutes...did that with my kids so thought I would try it.
Got through last night...she looks slightly better today but the breathing is sooo labored and she is exhausted.

Pray for her...my whole family is distraut and on watch...we took shifts through the night.

She was going to her new home today...of course the family is soo sad. If we can pull her through we feel we must keep her.

Any other suggestions are eagerly wanted...