I absolutely ADORE this song! However, this is what I think about whether or not it is Michael:
I'm a singer myself and for a good 2 years I've been studying Michael's singing style, also known as the "vocal hiccup". Now, I am no expert on the voice and how it works as I've only had a professional vocal coach for a year and a half, so if any of you know more than what I can tell you please feel free to correct me as I am still learning.
Anyway, on EVERY song Michael Jackson has done (as a solo artist beginning with Off The Wall), Michael has produced a little "hiccup" from his voice after every big breath of air. According to my vocal coach, he says that Michael probably does that to help him set up for a long note, to set up a new phrase, or to just add in a little something extra which makes him original. Now, as loyal MJ fans, we ALL know that Michael was known for his "vocal hiccup" technique. He did it on every song (beginning with the songs from Off The Wall) up until the day he "died". However, on this song, "All I Need", he didn't do it AT all. This tells me that this isn't Michael. However, there are times when at end of the vocal phrases where it sounds EXACTLY like Michael. I compared this song to We Are The World (Demo) and Speechless and to be honest, those phrases sound JUST like Michael, however, the long high notes do not. And because this singer doesn't do the vocal hiccup, I believe that Michael did indeed write the song, but in no way is this him singing. If it is, I will be totally surprised.
But I LOVE the song nonetheless! :D