Plutarch says in that very link:
"And now, in honour of Berenicé and Ptolemy, he gave the name of Ptolemy to his infant son by Antigone"

"By Antigone he had a son Ptolemy, Alexander by Lanassa, and Helenus, his youngest son, by Bircenna."

"Pyrrhus himself, then, with his men-at‑arms, tried to force his way directly against the many shields of the Spartans which confronted him, and over a trench which was impassable and afforded his soldiers no firm footing owing to the freshly turned earth. But his son Ptolemy, with two thousand Gauls and picked Chaonians, went round the trench and tried to force a passage where the waggons were."

Etc., etc.