The usual Quenya word for “son”, derived from the root √YON of similar meaning (PE17/170; VT43/37; Ety/YŌ).
Conceptual Development: This word dates all the way back to the Qenya Lexicon of the 1910s, but in that document ᴱQ. yondo meant “male descendant, usually (great) grandson” based on (archaic) ᴱQ. †yó “son” (QL/106). It seems the ordinary word for “son” in this period was ᴱQ. hilmo (QL/40, 106). These yo(n)- forms were also related to the Gnomish prefix G. go- “son of”, which showed the usual Gnomish sound change of initial y into g, but in the Gnomish Lexicon Tolkien changed {go- >>} G. bo- “son of” and introduced Qenya forms vô and vondo to match (GL/23, 40-41).
These early vacillations stabilized in the 1920s, since ᴱQ. yondo appeared with the gloss “son” in the Early Qenya Grammar of the 1920s (PE14/45, 75) and various word lists from this period (PE13/144; PE15/77; PE16/135). In The Etymologies of the 1930s Tolkien also gave ᴹQ. yondo “son” under the root ᴹ√YŌ or ᴹ√YON of the same meaing (Ety/YŌ). Yondo was used to refer to the “Son” of the holy trinity in Quenya Prayers of the 1950s (VT43/36-37).
However, in notes from the late 1950s or early 1960s Tolkien flirted again with replacing yondo. In one note he said “delete entirely yondo = ‘son’, very unsuitable”, perhaps because it conflicted with the root √YON “wide, extensive” introduced in those notes (PE17/43), but here Tolkien reversed himself and instead changed {√YON >>} √YAN “wide, large, extensive” (PE17/42). In a note from 1957 he wrote anon above yon-do as a possible replacement, and in another note he wrote “Q[uenya] wanted son, daughter” but without deciding on any new words (PE17/170). In yet another note from this period Tolkien gave yondo the gloss “boy” as well as “son” (PE17/190).
These vacillations again seem to have been mostly transient. In notes on Elvish Hands, Fingers and Numerals from the late 1960s Tolkien gave Q. yonyo as a finger name for the middle finger, variously glossed “big boy” (VT47/10), “son” (VT47/16) or “boy, son” (VT47/27). Thus it seems the sense “son” was restored to √YON, but with an alternate sense “boy” added.
Neo-Quenya: For purposes of Neo-Quenya, I’d limit yondo to mean “son” and use ᴹQ. seldo for an unrelated “boy”.
yondo noun "son" (YŌ/YON, VT43:37); cf. yonya and the patronymic ending -ion. Early "Qenya" has yô, yond-, yondo "son" (LT2:342). According to LT2:344, these are poetic words, but yondo seems to be the normal word for "son" in LotR-style Quenya. Yón appears in VT44, 17, but Tolkien rewrote the text in question. In LT2:344, yondo is said to mean "male descendant, usually (great) grandson", but in Tolkien's later Quenya, yondo means "son", and the word is so glossed in LT2:342. Dative yondon in VT43:36 (here the "son" in question is Jesus). See also yonya. At one point, Tolkien rejected the word yondo as "very unsuitable" (for the intended meaning?), but no obvious replacement appeared in his writings (PE17:43), unless the (ephemeral?) form anon (q.v.) is regarded as such. In one source, yondo is also defined as "boy" (PE17:190).