Joshua thinks dirtbikes are cool.
He'll learn and ride responsibly all his life and have so much fun sharing his sport with others. He'll become a strong self sufficient athlete all the while.
He'll learn acceptance and tolerance as well.
As mentioned upthread there are appropriate places for each activity. I don't know the area in question so I won't comment.
There are plenty of OHVers who are great athletes and aware of enviro issues. There are climbers who leave trash, cig butts, tape, and sh#t. Granted the average OHVer is less aware and will cause more impact that the average climber.
Pud's right ATVs are a way bigger problem than bikes. Any yahoo can jump on an ATV, while a bike takes some skill. There are way more injuries from ATVs because people are usually more experienced and careful on bikes. In Cali parks you can ride a bike without a helmet and can carry a passenger, while helmets are required on ATVs and no passengers (in response to the problems from ATVs).