Clear Creek Reservoir is probably a good option, there is free dispersed camping right near it: https://freecampsites.net/#!2446&query=sitedetails
The blogger RV Sue is there right now actually, looks like a nice place: http://rvsueandcrew.net/dispersed-camping-at-clear-creek-colorado/ (ETA: just saw her new post that said she left, but anyway, still looks nice).
The key there is that UTVs are not allowed. Many of the other boondocking spots I know are big with the UTV crowd so they probably won't be the experience he's looking for, especially right now in prime UTV season.
