You might gain a slight edge by changing out your wheels and tires for the lowest rolling resistance tires you an find, and slightly lighter and more aero wheels (assuming wind isn't the problem).
But, I'm assuming your nephew is in his late teens... and you aren't.
You may be able to turn the tables on the next race, but he's probably more motivated... and any advantage won't last long.
How do the two of you fare on hills? Perhaps add a mountain into he middle of your course, and see how the two of you do.
Are you actually training for these rides? How much riding do you do? The only way to really keep up will be to go out and pound the pavement hard, 3 or 4, or more days a week. Add in a few longer rides. 50+ miles?