Are both the same?
Are both the same?

I met him for once.
I met him for the first time.

And, does "I met him for the first time" mean,
I won't meet him again.

Question

Are both the same?
Are both the same?

I met him for once.
I met him for the first time.
  
  And, does "I met him for the first time" mean,
  I won't meet him again.

Answer

They are not the same.

I met him once can be used as past tense. Met is a past tense of meet. Or can mean that you only met him one time. You may meet him again.
I met him for the first time is meaning that you just met a person. It doesn't exactly mean you wont meet them again unless you choose that.