Someone who never gets tired

Question

• He always works and I have rarely seen him sitting. A hardworking man who never tends to show down or stop working. He always works with lots of energy and effort and never gives up while he can achieve what he wants. He is really a/an............. man.
  

I know four words here which seem to be quite synonymous to me. But I don't know how they sound to you and which one fits better here in my example.

Here are the words with their definitions! Please have a look on them and let me know which one(s) is/are the proper word(s) and why?

Also, please let me know about the possible slight nuances between them.

a. tireless
b. untiring
c. indefatigable
d. relentless

I think, all the first three work interchangeably here, and the only nuance of them would be the higher formality of 'c'. Regarding 'd', I think has a little deragetory (disapproving) connotation involved.

Tireless [approval]

• working very hard and energetically and continuously without stopping refusing to give up or take a rest in a determined way.
  — Tony is a tireless worker.
  — The police have been tireless in their search for the child's killer.
  

Untiring [approval]

• Somebody who in order to do something continues to do it with a lot of effort or energy without stopping.
  — An untiring fighter for justice.
  

Indefatigable

• Someone who never gets tired of doing something. Always determined and energetic in trying to achieve something they want and never admits defeat.
  — His indefatigable spirit helped him to cope with his illness.
  
• He was indefatigable in his efforts to secure funding for new projects.

Relentless

• Someone who is relentless is determined to do something and refuses to give up or stop, even if what they are doing is unpleasant or cruel. — He was the most relentless enwmy I had seen.

P.S. bringing up this question, I was going to discover how these words differ in meaning and where can each one be used?

Answer 1

Your analysis is quite correct.

a. tireless
b. untiring
c. indefatigable
d. relentless 

Tireless means unstoppable with your approval;

Untiring means unstoppable but this word is not quite as common as "tireless";

Indefatigable means the same but sounds formal, mostly because it's a six-syllable word;

And Relentless suggests either disapproval, grudging approval or approval as the person faces difficult opposition.

1. The bully has been relentless in tormenting my son.

2. President Smith has been relentless in his quest to discover and fire his political enemies.

3. Our troops in the west have been relentless at discovering and fighting the enemy.

Notice in #2 above that "President Smith" is doing an activity which is not really of a physical nature. You likely wouldn't use "tireless" in this case, because tireless implies a physical activity that makes you tired. "Relentless" can be used for this purpose (#3), but it can also simply mean that the person has not relented (stopped or slowed down).