Why does Jesus call Judas the "son of perdition" in the Gospel?
Commentaries on John 17:12:
§ f 12–14. Up to the present, Jesus has kept them, so that none of them fell into such temptation as brought permanent rejection (cf. 18:9), except ‘the son of perdition’—a Semitism of metaphorical relation denoting destiny, like ‘son of gehenna’, ‘son of death’. Judas was destined to perish, but by his own fault. The Scripture had foretold it, Ps 40(41):8. The treason of Judas, foreseen as the result of the traitor’s own malice, was an element in the plan of redemption. This prayer spoken by Jesus in the world is to secure for the Apostles a full participation in the serene joy which he himself had in the accomplishment of his Father’s will. The professors of heavenly doctrine will surely be hated by that world which holds to an earthly philosophy.
Source: Leonard, W. (1953). The Gospel of Jesus Christ according to St John. In B. Orchard & E. F. Sutcliffe (Eds.), A Catholic Commentary on Holy Scripture (p. 1010). Toronto; New York; Edinburgh: Thomas Nelson.
Ver. 12. While I was with them, I kept them in thy name. He still speaks, says S. Chrys. as man, and after a human manner, by mentioning the advantage they seemed to enjoy, as long as he conversed visibly with them on earth, not that his invisible presence should be less beneficial to them.—And none of them hath perished, except the son of perdition, the wretched Judas, whose fall was foretold in the Scriptures. Psal. 108. He hath perished, that is, now is about being lost, by his own fault, says S. Chrys. on this place. And S. Aug. on Psal. cxxxviii. How did the devil enter into the heart of Judas? he could not have entered, had not he given him place. Wi.—That the Scripture may be fulfilled: this does not any ways shew, that it was the will of God that Judas should be lost; but only that what happened to Judas was conformable to the prophecies, and not occasioned by them. Who will doubt, says S. Aug. (lib. de Unit. Eccl. c. ix.) but that Judas might, if he pleased, have abstained from betraying Christ. But God foretold it, because he foresaw clearly the future perversity of his disposition. Calmet.—See above, (13:18) one of the principal passages of Scripture relative to the treachery of Judas, in which the traitor’s crime had been predicted.
 Haydock, G. L. (1859). Haydock’s Catholic Bible Commentary (Jn 17:12). New York: Edward Dunigan and Brother.