It helps that Inigo saw Senku's world back during those few days of being lost out there. It definitely makes it easier to reach out towards Senku in the void after having been dragged back to the train himself. Ending back up on the train after all that was tiring enough that Inigo felt it better to be the one staying on the train, providing the stationary anchor that could hopefully draw Senku and Tidus back there.

So he just tries to focus the best he can. Thinking about Senku until he can sense that Tidus must have arrived at his destination, though with that groaning...

'Are you okay?' is Inigo's main thought at that, sending it over towards Tidus. But if Senku focuses enough, then he might be able to hear it too - a voice somewhere in his head, reaching out to him from further beyond.