''poika'' is like Swedish ''pojke'' in this sense. Did Finnish adopt this word? If so, why? and what was the original Finnish word for 'boy'?
The Swedes actually loaned the word from Finnish! Many Finno-Ugric languages have similar words for "boy" or "son" (for example "poeg" in Estonian).
Could this possibly be the origin of the English word "boy"? They sound very similar, but I guess this may just be coincidence!
It's probably just coincidence. The origins of "boy" can be traced back to the Proto-Germanic word "bōjô", while "poika" originates from Proto-Finnic.
Poeg has also been borrowed to southern swedish as "påg" which also means boy.
To my Hungarian ears "tyttö ja poika sound so so cute.