I visited Merwin, MO today and rode for an hour or so on the trails. I have to say, I'm excited to find this place. It's been in business for many years, but somehow I never heard of it.
It's primarily a motocross track, but they have a few miles of trails, too. The motocross track looked like a really nice one, but I don't know that much about MX. When I was there today, 3 guys were playing on the track. I was the only guy on the trails. They have a second MX track that is smaller and designed for kids.
Cost is only $10 for a day. On race prep days, it is $20. Normally only open on weekends. Hours are normally 9 until dark, but you may want to call to ensure they are open.
The trails were really fun--sure, it's not Chadwick, but there are tight woods, a creek, a small hill climb, and some sweeping, open stuff with berms. At less than 40 minutes from my house, I know I'll go back to play. It would make a great day of dualsport--ride the gravel roads down there, play for an hour or so on the trails, then dualsport home.
When you go, please tell him you heard about the place on RideForum.NET.