The front legs and chest down, rear end up in the air position is called a prayer bow and is often an indication of abdominal pain in a dog when he is showing other signs such as restlessness and inability to settle. It is a stretching motion attempting to relieve pressure/pain in the abdomen.
In cases where the dog acts uncomfortable it is likely that the pup is experiencing some reflux or abnormal motility, or painful intestinal spasms that are more than the usual. In severe cases this can be linked to pancreatitis which is a painful inflammation of the pancreas leading to increased enzyme leakage.
Possible causes of nausea and abdominal pain include a change in diet, dietary sensitivities or allergies, or eating things that they should not like too many fatty table scraps or garbage, bones etc. Addison's which is a poorly functioning adrenal gland is another possibility for waxing and waning vomiting and nausea. Metabolic organ failures (kidney or liver disease), pancreatitis, inflammatory bowel disease or even infiltrative cancers are possible causes as well.
Has he eaten anything he should not have recently (toy pieces, bones, garbage)?
Any changes in food or treats?
You can give him an acid reducer to try and settle his stomach. Either:
1) Pepcid-ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle his stomach and reduce abdominal discomfort. These can be used for several days if necessary as they are very safe.
I'd also pick up his food and water for now. A couple hours after one of the acid reducers you can offer small amounts of water or ice cubes to lick.
No food for 12 hours. Small amounts of water only.
After his food fast then start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled white rice. Give small meals several times a day. Feed the bland diet for several days, then if all is going well start mixing in his regular diet and slowly convert him back.
If his nausea and abdominal pain continues then he may need injectable medication from his veterinarian to get his stomach upset under control.
So watch him for continued symptoms even with the acid reducers, blood in his stool or vomit or a fever (more than 103.5F rectally), a tense painful belly or lack of appetite after his food fast. If any of those occur it is time to seek hands on veterinary care.
If his symptoms continue it would also be a good idea to have blood tests done as well to make sure there isn't an underlying metabolic problem like early organ failure, pancreatitis or Addison's disease present.
If he is a larger breed, deep chested dog, his abdomen is rapidly enlarging, he tries vomiting and is unable to do so or is repeatedly retching then he should be seen immediately at an emergency clinic as he may be suffering from bloat, a life threatening emergency.
Please let me know if you have any further questions.