Its not months its 10 years he'd have to grow if he is Xehornort so it is possibly he could grow a whole head
Um, no.
That cutscene in KH2 takes place months to a year after BBS, which is ten years before KH1.
And by that time, Xehanort continues on with his experiments and turns into Xemnas and Ansem SoD and is like that for nine to ten years until KH1 begins.