I have to wonder if this is just a matter of perception. Granted, I don't know the specifics of your son's Rockhopper, but in looking up both bikes, they're very similar in weight (roughly 27 pounds) depending on size and model. Your Fargo seems like a good compromise based on the things you're looking for in a bike. There's a good chance that an aluminum bike would ride rather harshly on the bad roads that you have to deal with. If you really want to lighten up the Fargo, look at getting some lighter wheels. The frame itself is probably not what's really slowing you down. It's your rotational weight.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher