Yes and No. You have to link the two accounts together in the Xbox One settings. You will not have to buy a monthly Xbox live service. Your grandson can use your linked account to play the online game. You will have latency issues though. If you wanted to play the game, you would have to create a VPN, get NO-IP to resolve the IP address lease issues for the VPN (which some modem routers support) and away you go.
On a side note, I have found a bug or feature with this method to circumvent the required migration of a non US Xbox with a US based xbox one live account and keep functionality of all bought online games stored on the non US Xbox One or 360. Where the DRM stopped you previously, you can pass this but only on linked accounts. Non US games are still region locked. If you have already bought the games, why buy it again or have your Xbox 360 modded to allow it?
Some help once you have your VPN sorted.