I know this is an old post but I had to say something to these replies.
I love how people who don't know parrotlets at all and are scared to let their bird fly feel the need to come up with stories to scare others away from letting an animal do what it naturally needs and wants to do.
Like that one comment telling you about how a smaller bird will scare more easily.
That person very clearly has never had a parrotlet and knows nothing about them. They are big birds in little bodies and most definitely do not scare easily.
Parrotlets are also extremely intelligent. They can certainly be trained just like any other parrot. They are, after all, "true parrots". Look them up. They can even learn to talk.
There are dangers, but the dangers of a small bird in free flight are offset by this bird's speed and maneuverability. They are among the fastest birds in the world and can fly like humming birds. They're actually being studied for drone flight. When my parrotlet's wings were clipped (when I first got him) he could still fly anywhere he wanted in the house.
I think these people just envision the worst about free flight and don't know anything about it, so it scares them. As long as you go to a big open field where birds of prey don't live, you should be fine. Just make sure he's recall trained!