Sorry, but it's not possible. If his friend has a Wii-u, he could plug it into the computer, and play there, but he needs to buy a Wii u. But, if his friend could come over to your house, you could buy a secondary controller for Wii u (10-30 dollars) and they could play together on the Wii-u. Hoped I helped!
(P.S: If your son is lonely on the Wii-u Minecraft, my son's NNID is Pokerfacememez.)