I simply ask to observe as you fix it, and to have my questions answered during the process, should I have any. [He just wants to see a hands-on demonstration on any ways he himself can do better. "Teach me everything you know ever" is a bit of a tall order to start with, even Sebastian realises that...]
Would that be acceptable? I will of course pay more handsomely than if you should work undisturbed by me.