From what I can see, the etymology for "boy" beyond relatively modern cognates is still undetermined. It could have entered germanic before 900 and left no written traces before later appearing in Middle English. Finns and Estonians were involved in sea trade around that time (borrowing words such as kauppa, cognate with "shop") so a word for "servant boy" would have been useful.

In particular, the pronunciation of Estonian "poeg" and "poiss" seem too close to be entirely coincidental.