It says 4 comments but there appear to be only two.
But, anyway, my main point is that this is fiendish. It moves immediately from instrumental synem to the next two words in genitive?
Well... yeah. The sentence is built as "This is Y", right? And the noun phrase Y is "my neighbour's son". If we consider this noun phrase on its own, it's "syn mojego sąsiada". Saxon Genitive in English = Genitive in Polish. Now we're putting this Y into the sentence and we see that Y must take Instrumental. And luckily the Genitive part doesn't change anymore here, you just need to put "syn" in Instrumental. "synem mojego sąsiada".
"He is my neighbor's boy" isn't acceptable? It's not quite a literal translation, but it's functionally identical.
Still, "boy" is not a translation of "syn". And that's definitely the first such suggestion I have seen in more than two years.
Respectfully, I'd argue that it is:
Respectfully, I apologize, seems that you are right. However, I'd still prefer to keep the translations as literal as possible, especially that you're really the first person to suggest that. As 'just acceptable answers' (not the main, starred ones) sometimes get suggested without any particular reason, I don't want anyone to get such a suggestion and get confused. Especially people that are not native speakers of English could get confused about the exact meaning of "syn".