An obvious gesture, definitely good enough. They absently start stroking along the top of his skull in response.

"...kay. And you won't run out of food. And I'll visit." Frisk sounds distant, lost in their thoughts of lab stuff. Most of it isn't all that pleasant, but they're not going to outwardly show it. Not when there's someone who was already hurting nearby.

They suppose they should leave him to his nap...but they also suppose they'll wait until he's actually asleep. Company is good. And even if he doesn't remember anything, even his name, he'll probably end up missing his brother.