What could this be, is it urgent?

I am taking in a stray dog.  He is extremely skinny.  He eats constantly, and poops a lot, 3 or 4 times per day.
He is very weak and unstable when standing or getting up, especially on his hind legs.  
Is this common?  Is it urgent?  

[Aleda M Cheng, D.V.M., C.V.A]

The common causes of the symptoms you have described are gastrointestinal parasites, or protein losing enteropathy (PLE), or even Addison's disease.  

But there are many other possibilities.  It really would be best to take him to a vet to get a definitive diagnosis.  The vet can examine him, perform digestive profiles, parasite checks, neurological exam, bloodwork, and any other tests deemed warranted.  

Please bring him to the vet ASAP!  Please let us know what was discovered.