What extra words does he say?
Rashi #1: Bil'am glorified himself, and called himself "Yode'a Da'as Elyon" (Bamidbar 24:16).
Rashi #2: He says, tomorrow I will do so to Ploni. He does not know what will be! Ri Kara - he speaks as if it is in his power to do so.
Rashbam: He says many words of folly, and does not know the evil that will come from them.
Rid: He does Aveiros and evil deeds, he says many empty words, and says in his heart 'man can do what he wants.'
What does he not know about the future?
Rav Sadya Gaon: He does not know what will result from his words.
Rashi #1: Bil'am went to get his wage in Midyan. He did not know that he will be killed via the sword.
Rashi #2: He does not know if he will be able to do to Ploni like he desires.
Ibn Ezra: The fool says, I do not know what will be in my lifetime and in my death, [therefore I will eat and drink]. Perhaps this is like "Ki Yesh Devarim Harbeh Marbim Hevel; ... Mi Yode'a Mah Tov la'Adam" (6:11-12).
Rid: He does not know if Hashem will punish him in his days.
Seforno: This is his reply to a Chacham's rebuke of the damage that he causes via his folly. No one knows what will result from my folly!
Metzudas David: One who hears the fool's words, he does not know the purpose of his words or his primary intent.
What do we learn from "va'Asher Yihyeh Acharav Mi Yagid Lo"?
Rashi: Not only does he not know what will be later. Even what is now in back of him, not in front of his eyes, he needs someone to tell him!
Rashbam: The evil that will come to him suddenly from in back, when he goes on his way, he does not know it, for there is no one to tell him. Therefore, he should distance from folly.
Ri Kara: If he does not know what will be today, all the more so he does not know what will be afterwards!
Rid: He does not know if Hashem will judge him after death; he is in doubt about it.
Seforno: When a Chacham tells him not to breach the fence more, lest a snake bite him, he says 'one cannot know the future.' He thinks that the intellectual Nefesh is like the animal Nefesh, which feels only what is sensed.
Metzudas David: Even if a fool will tell Ploni about something in back of Ploni, not in front of his eyes, Ploni does not know, even if it is easy to understand. Who can tell him? A Kesil has no speech or words [on which one can rely - PF].
