Whilst it is common for the terms 'Proprietor' and 'Director' to be used inter-changeably, technically they are statutory terms and their use is strictly governed by the Companies Act and other similar legislation.
A Proprietor is a person who owns a Proprietorship. This is the simplest and easiest form of business registration. In this case, there can only be one owner. Usually, they will have a trading name on the basis of which they will acquire the business PAN card, service or sales tax registration etc...
A Director is a person who controls a company - whether private or public limited. They have signed a Memorandum of Directorship, have stringent statutory obligations placed upon them. And, remember, a company MUST have at least 2 Directors.
So, the easiest test re: whether to call your boss a Director or Proprietor is this: If his business is a Private Limited company, then he/she is a Director. If it is a single-person owned entity with a trading name attached, then he/she is a Proprietor.
And, to bring further clarity, if your boss is in a registered partnership, and is performing most of the business' management work, then he/she is a Managing Partner.
Hope this helps you.
Take care!