People can’t join my friend’s world!

We’ve been playing on this world for about two weeks regularly until one day when I couldn’t join his world.

He plays on Switch, we’ve tried to connect on a PC and an XBOX and it simply says ‘unable to connect’ after a few seconds of loading.

He can, however, join other friends worlds so the problem must be on his end.

Does anybody have any possible solutions? I don’t really know how to fix things on a switch.

We’re playing Minecraft 1.12.