The usual patronymic for “son of” in Quenya, suffixal form of Q. yondo “son” (PE17/170, 190). Tolkien occasionally mentioned variants like -on or -yon, but in practice only -ion appears in actual names.
Conceptual Development: This patronymic dates all the way back to Qenya Lexicon of the 1910s (QL/106) and was regularly mentioned in documents throughout the years such as the Early Qenya Grammar and English-Qenya Dictionary of the 1920s (PE14/45, 75; PE15/77), The Etymologies of the 1930s (Ety/YŌ) and Notes on Names (NN) from 1957 (PE17/170), always with a similar form, meaning and derivation from roots likes √YO(N). Thus it was very well established in Tolkien’s mind.
-ion (patronymic ending) "son (of), descendant" (YŌ/YON, LT1:271, LT2:344). Not to be confused with the genitive ending -on when added to words with nominative plurals in -i, e.g. elenion "of stars" vs. eleni "stars".