Welcome! My name isXXXXX am a UC Davis graduate, and currently, a Medical Director of a veterinary hospital.
I am sorry to hear about this concern for Angus.
Very difficult question.
Unfortunately, lethargy and/or weakness simply has too many differentials. We don't know if this could be secondary to some type of infection, possible pain, a endocrine condition like Addison's or hypothyroidism, to name a few.
So, if it is very obvious his change in behavior, than you know for sure there is clearly a systemic (internal) issue that needs to be addressed.
So, what to do?
I would schedule a examination, even tomorrow if really appearing weak, and request some basic testing.
A canine basic blood test and a urine test (urinalysis) may answer a lot of concerns, or at least rule-out many possible explanations.
What can tests tell you:Chemistry Panel CBC (complete blood count)Urinalysis
Otherwise, without the tests, there is just no way for the vet to guide any guess at therapy.
I hope that information has been helpful.
Please remember to select the tab, REPLY TO EXPERT, if for any reason you need further clarification, have more questions, or were expecting a different type of answer. Prior to leaving, please remember to select the RATE TO FINISH tab to provide a feedback rating, which I greatly appreciate so I am recognized by Justanswer for responding. My goal is to provide you the best answer possible.