Am i legally using copyrighted song, if the singer of the song agree me to use his song, and what he just need is just a credit in my game description,
What i did is, i asked him via facebook for legal using his music, and he is totally fine, but should i do more? Should i apply for a permission or some signature to proof it? Thanks for the help
Okay: 
I know in some cases, if the singer is also the composer of the music track
and he gives you the okay, to use his tune in your game, it seems perfectly fine.
But, if the singer is not the composer of the tune, it means that you have
to get permission from the creator/composer of the music track.
Also, sometimes the record label, can have a hold, or part ownership
or worse…full ownership of the music track too.
Which means, that if even you got permission, from the singer and composer
of the tune, you still have to get permission, from the record label too.
And unfortunately, most of the time, they might not agree.
Or they might agree, if you are unfortunately asked by the record label, to
advertise their record label, in your game too. 
Which of course, most game designers would (smartly) refuse to do.
Since in some cases, that could later on, turn out legally nasty and icky in the end.
Tip: Ask the singer, this question.
“Who exactly is the composer and full owner, of the track?”
If it’s the singer who already granted you permission, to use it in your game
then go ahead and enjoy.
But if the record label and singer/composer, all have ownership, or it is owned
fully by the label, then you’ll need permission from the record label too.
Note: In some cases, the record label could be owned by the singer.
Which would make it easier, in your case.