What else have you done to the bike?
Clipless pedals are a good suggestion, because you're attached to the pedals at any part of your stroke. If you aren't wild on that, you could always get a set of clips and straps to get a similar result. You could also add some bar ends to mimic bullhorns and make climbing easier/faster. Another possibility would be flipping the stem. This may or may not actually help you get anywhere as far as speed, but it could theoretically help you get a little more of a tuck to cut down on wind resistance.
Aside from that, make sure your tires are chock full of air, and it wouldn't hurt to let some air out of your brother's tires. If that doesn't work, make him drink a big glass of milk before each ride.