Whose initial words are discussed?
Rav Sadya Gaon: The Kesil's.
What is the source that his first words are folly?
Rashi: When Hashem asked about Bil'am "who are these people with you?" (Bamidbar 22:9), he should have said 'Hashem, You know!' Rather, he said "Balak ben Tzipor Melech Mo'av Shalach Elai" (ibid., 10). I.e. [even] if I am disgraced in Your eyes, I am esteemed in the eyes of earthly kings 1 .
Lev Eliyahu Shemos p.276): This is lunacy! What is a mortal king compared to Hashem?! Would one tell his friend 'you do not esteem me, but the cat esteems me!'? Refer to Bamidbar 10:22:151:1-2 and the note there.
What is "Holelus"?
Rashi: It is madness and confusion - "Lechah I'Atzecha" (Bamidbar 24:14) - counsel of Zenus and folly.
Ibn Ezra: It is without reason - his initial and final words have no reason.
Do we know about his words in the middle?
Ri Kara, Metzudas David: He does not cease speaking folly, until he causes evil to himself.