I'm sorry to hear that Mace has a cough and is spitting up white phlegm and I understand that you are concerned about him.
A cough simply means he is experiencing airway irritation.
A cough can be related to heart disease, including heartworm, lung disease secondary to a viral, bacterial, fungal or parasitic infection or tracheal irritation due to a bacterial, viral or fungal infection.
In a young dog infectious disease is by far the most common cause. Of those kennel cough or infectious tracheobronchitis is one of the most common causes. Because we know that he was at a campground where other dogs were likely present, or had been around recently within the previous two weeks ago kennel cough would be high on my list of possible problems.
Even if he has been vaccinated we cannot vaccinate against all of the agents that cause kennel cough and some dogs will still get sick even with the vaccine. The good news is that dogs that are vaccinated usually don't get as sick as dogs that haven't been vaccinated.
As long as he is still able to eat and drinking well, isn't lethargic, isn't running a fever (rectal temperature greater than 103F), and he doesn't have a green or yellow nasal or eye discharge then you can try taking a watch and see approach until his schedulaed examination on Thursday, and using an over the counter cough suppressant if he is coughing so much he cannot sleep.
Make sure he gets plenty of fluids, lots of rest as the more active he is then more he will cough, and eats well. Add warm water or low salt chicken broth to his food. The better hydrated he is the better he will feel and the thinner his respiratory secretions will be.
You can take him into the bathroom with you when you take a hot bath or shower so he can breathe the steam which is soothing to his airways and thins mucous, making it easier for him to breathe. You can also use a humidifier in the room he sleeps in,
With a usual kennel cough infection his cough should slowly resolve over the next 2 to 3 weeks. Most of the time these dogs feel pretty well so it's a matter of keeping them quiet and suppressing the cough if we can.
If the cough is very bad you can use human cough/cold medications with Guaifenesin (an expectorant) and Dextromethorphan (a cough suppressant) only such as Benylin or Robitussin DM can help. Give 1 teaspoon per 20 to 25 pounds every 6 to 8 hours.
If he is running a fever this is almost certainly a contagious infection and if he is running a fever, refusing to eat at all, and is lethargic then an examination as soon as possible is definitely warranted as pneumonia may have developed.
Other viral infectious possibilities for a cough include influenza. Those dogs can get very sick very quickly so if his fever is high (rectal temperature greater than 104) and he is very lethargic he should be seen on an emergency basis.
Best of luck with your fellow, please let me know if you have any further questions.