A person who is adamant because he knows he is right, without being rude [duplicate]

The word authoritative has meanings which can be combined to get the sense that you are looking for.

ODO:

authoritative ADJECTIVE

1 Able to be trusted as being accurate or true; reliable

‘Clinic staff may be seen as authoritative sources of prevention information.’

2 Commanding and self-confident; likely to be respected and obeyed:

‘his voice was calm and authoritative’

Addendum (courtesy of Joe Blow):

And the good news is,

"The authority"

is a commonplace catchphrase, often used in the US.

If you say to your boss...

Boss, you are the Authority, dude.

Your full sense is conveyed.

Examples of use ... "Here comes The Authority" (as he walks in to the room) or "We need to ask The Authority - our boss" (when a problem arises).

This phrase has that exact "pumped-up / sports-crowd like" feel that US audiences look for, or indeed need. So, when you exclaim, "Boss, you are The Authority!" it could well be followed by everyone in the room fistpumping, doing the deep "yeah", and then chanting U-S-A-U-S-A.

Note that indeed it is used in a number of marketing slogans; "XYZ Sports stores .. the Sports Authority" and so on.

Use in a marketing tagline is the ultimate proof positive (in the USA) that the phrase is absolutely - utterly - wholesome; spotlessly positive; spostlessly politically correct; and very broadly understood. So, you're golden.

steadfast

: firm in belief, determination, or adherence : loyal

Steadfast is often used to describe storybook heroes — The Steadfast Tin Soldier is a notable example — but the word is commonly understood, and appropriate here.

To call someone steadfast would always be a compliment, although its heroic tone might make someone chuckle.

Usage example

I am impressed with how you handle the pressure to take unwise shortcuts. I only wish I could be so steadfast.