This is quite a personal poem that is obviously directed at someone you know. I love the venting aspect of this poem, raw, emotional, and just plain bitter.

I wonder who it is that you speak of...

x

Who always claims
That he could ace the test
When he would have really put his mind to the test?
He does, he does

--

I would suggest to lower case "That" so that we don't make any stop when reading. Also in my opinion, repeating test in one stanza might not be the best way to go.. perhaps find a synonym that rhymes with Test? But ah, I was looking for some and I couldn't find one that fits the context just the way you had it..

then perhaps say When he really puts his mind to the test?
without using the would have.. since I think that would show that this guy is showing his pride.. or confidence at full power.

Here is my suggestion,

Who always claims
that he could ace without the need to retest
when he really puts his mind to the test?
he does, he does.

--- hehe perhaps not the best, but by having the re in front of test adds a different sound. And it doesn't stress the same one as test and test. I don't know perhaps not the best suggestion, feel free to ignore it.

Who claims
He could have been
The white Michael Jordan?
He does, he does

Who always claims
He knows all the music
Especially the one you never heard?
He does, he does

--- I think this two stanzas would benefit if you combine them together... like this...

Who always claims
that he could have been
the White Michael Jordan
or that knows all the music
specially, the one you never heard?
he does, he does.

or in any other way you prefer. Repetition can be our best friend but it can also get over our heads.. and that's what the he does, he does, did to mine. That's why I feel it would be better to just combine them. But that's just me. I don't know if it happens to other people.

I will combine all my suggestions at the bottom of this comment.

But this is the present
No way to know
What the future
Will hold in store

-- I think it would sound better if you add There is ... to no way to know...
and instead of hold in store.. change to for us..

But this is the present
there is no way to know
what the future
will hold in store

Who will fail
At the life he envisioned
Cause he is too damn lazy?
He will, he will

-- Then on this one, I feel that it would be better to say could instead of will since on the previous stanza, you stated that there is no way to know what the present will hold in store for us.. So how can you be so certain that he will? So instead, just change will to could.

Who will wonder at night
If he made the right choice
Marrying his first love?
He will, he will

--- Adding by in front of marrying could make it sound better..

Okay, aside from my suggestions, I just feel the need to say that I wish this is just pure fantasy and that you don't wish this to this person. I know, people who feel so great can be annoying at times, but really. You are just going to hurt yourself by wishing to see this person's future go bad. I also hope that I am misunderstanding your poem and that instead you wrote this poem with the intention of opening that guy's eyes. And for him to stop being arrogant.

--- Okay, here is my entire suggestion put together, Feel free to use any if you like them. If not feel free to ignore them.

Who always claims
that he would have ace without the need to a single retest
when he really puts his mind to the test?
he does, he does

Who always claims
that he could have been
the White Michael Jordan,
or that knows all the music,
specially, the one you never heard?
he does, he does

Who always brags
about his girlfriend
'Cause she's the first one he ever had?
he does, he does

'Cause he is
the one and only
The best, the bravest, the smartest.
Mister, I'm so great.

But this is the present
there is no way to know
what the future
will hold for us.

So who could fail
At the life he envisioned
'Cause he is too damn lazy?

Who could wonder at night
if he made the right choice
by marrying his first love?

Who could tell tales
of the things he never did
to his uninterested kids?
he could, he could

Cause he was feeling
So great
That he could not see
The future was coming
his way

And if it happens
I will salute him
With a smile as big as it can be
Mister, I'm now not so great.