Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I am sorry to hear about Wicket's lack of appetite, lethargy and difficult going up and down stairs.
A sudden lack of an appetite in a dog can be from several causes. Common ones are a quick change in diet, too many treats, especially fatty or spicy treats, eating things they should not such as things found in the home including rubber bands, toxins, toy pieces or garbage, or viral, bacterial or parasitic infections.
More serious causes of vomiting include organ failure, diabetes, inflammatory bowel disease or even a mass in the abdomen (all much less likely in a young dog but I mention them to be complete). It is also possible that he is painful enough from some sort of back injury that he isn't eating well.
Did he have an abrupt diet change recently?
Is he current on his vaccinations?
Has he had a stool sample checked for parasites recently?
Even if he isn't around other dogs you can bring things home on your shoes and clothes and expose him to viruses and parasites without knowing it.
I am glad to hear that he has an appointment this after noon. Whether this is an emergency now depends upon his general condition.
If she starts vomiting, has a tense painful abdomen, very pale or white gums and tongue or runs a fever (more than 103.5F rectally) then he should be seen sooner. I know you aren't with him now but did you notice any of that this morning before you left him?
In the meantime to try and settle his stomach when you get home today you can try either:
1) Pepcid-ac (famotidine) at one 10mg tablet per 20 to 40 pounds of body weight
every 12 hours.
2) Prilosec (omeprazole) at one 10mg tablet per 20 to 40 pounds of body weight
every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation. These medications can be used long term as needed as they are quite safe.
I would not feed him any food for 24 hours after the acid reducers are started. This should help stop gut spasms and restore normal gut motility in case this is related to simple stomach upset. Small amounts of water or ice cubes given frequently are fine as he needs fluids. You can give him pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make his intestinal irritation worse.
After his food fast start a bland diet of 1/3 boiled hamburger (or white skinless chicken), all fats and juices drained off mixed with 2/3 boiled, plain white rice.
Once he feels better (good appetite for 48 hours) start mixing in his regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert him back.
If Wicket has a tense painful abdomen, vomits even with the acid reducers, becomes very lethargic, has pale gums or tongue or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible.
Please let me know if you have any further questions.