[... Looks embarrassed. M- mrr okay, so it probably has to be a promise he actually means, then. Damn it all.] [Almost promises not to hit him for the rest of the day, but Merlin would undoubtedly abuse such a promise :| And promises to carry him wherever he wants or to kiss him whenever he wants or to let him take a bath all seem kind of obvious since he lets him do that anyway...]
[...]
[S- sigh and looks away, EMBARRASSED FOREVER and thinking this is probably going a bit overboard since he could probably promise something like, to not ever steal dresses for him or something, but.]
I promise that I'll do everything in my power to make sure you're happy here. [If only a little, etc etc.]