Techniques to help with keeping kid's focus during guitar lessons?

As the title says, I teach guitar. Ive just taken on a 7 year old boy who was awesome during the first two lessons, but i must have given too much leeway and made him too comfortable because in our last lesson he literally spent about a third of the time that i was there chasing a fly around the room and another third with general goofing off.

SO my question is what are some tried and true ways i can keep in mind to keep this little bastard focused in my next lesson with him? I want to keep it fun because once lessons stop being fun is when people generally stop playing instruments, but his mother is paying me good money for this and I need to be sure she is getting her money's worth while still keeping it a somewhat enjoyable experience for him. Maybe my expectations are too high and i should only expect about 15 minutes of actual work per lesson? That seems a bit ridiculous honestly, I mean a bit of horseplay is good but that is definitely excessive, right?

Let me know what you guys think and thanks a bunch in advance!!