com is a memorie or dream of his that we play

Sort of, as far as I know the point of the game is to reclaim his lost memories. Its possible in that case that the whole thing is a battle inside Sora's psyche, though so far there isint a whole lot of evidance of that that I have seen.

I read somewhere that in CoM, everyone who was Sora's friend in KH forgets who Sora is. so he has to retrieve his lost memories as well as everyone elses.

It's pretty much just Sora getting his memories back, but there must be something more because Axel(while in hood...lord knows how he shoves his hair in there) said something to Sora about not being the same person after he remembers everything or something like that...