Thanks for the additional information! From the sounds of it, he could probably use Riverview's version of the Astral Dimension - essentially, as long as he can't use it to travel back home or to other characters' worlds, and it only works within the context of the reality the Quarantine is in, it should be fine.

Here are a couple of relevant answers to previously asked related questions that might give you a clearer idea!

Welcome to the game!