What does Islam say about kidney transplant from non-Muslim to Muslim boy in non-Muslim country?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

For Shari'a ruling concerning donation and transplant of body parts, check our Fatwa: 82240.
Anyway, as for your question concerning 'kidney transplant' from non-Muslim person to Muslim person in non-Muslim country, such a state does not make any change in Shari'a ruling as long as an operation is done according to what is mentioned in the afore-said Fatwa.
Allah knows best.