It depends in which country he is.
In England, a physician is usually a general practicioner (GP), with no specialisation. An endocrinologist specialises in things related to the endocrine system (glands and anything else related to hormones).
In America, they are all called "physicians," whether they specialised or not. The ones who haven't specialised are usually called "family physicians" or something along those lines. Not all physicians are endinocrinologists, but all endocrinologists are physicians.
Having only one "assistant" is pretty common for a specialist, as they treat only a small subset of the people who normally go to GPs (or family physicians). His/her "shop" is called an office

You usually need to be referred to an endocrinologist by a general physician if you want your health insurance to cover its portion of the bill.
Their office door usually tells you what your doctor's specialisation is. If not, look on the walls inside the office -- their diplomas are usually framed and hung there. Of course, you could always just ask him!!!