If he's sleeping it means he's finally comfortable in his new environment. Don't mess with his sleep. Young FRTs float to the surface every couple of hours for a breath before settling back down for more sleep. As they grow, they'll take longer naps between breaths. My FRT is large enough that we can hear him periodically take a breath from the other side of the house, every few hours, after he starts napping in the late morning. Mine no longer buries himself. He lies in his large open sided flower pot and just rests his flippers on the substrate. He'll rouse himself from his sleep if I approach the tank because he's expecting a treat (of which I'll give him).
When he's active, if he sees us giving the cats treats, he'll start splashing at the surface to get our attention so we don't forget him during treat time.