You need to buy the new Minecraft digitally, unless your son has played for 5+ hours between Sep. 2016 and Sep. 2017. Since servers seem not to be available, check for updates. If after any updates (if there are any), it still doesn't work, it means you do not meet the criteria and must buy the digital version.
If you own the disc version of Minecraft: Xbox One Edition and you
have played for at least 5 hours or purchased an item between
September 2016 and September 2017, you will also receive a digital
copy of Minecraft under Ready to install. This offer is valid until
January 31, 2018, after which time you will need to purchase a digital
version of Minecraft.