No, he isn't sprinting off never to return, he's chasing his basketball. We take it to make sure we can get him back once he's off and to wear him out. It's great!
He loves being off lead! He can run a lot further with the ball and say hello to other dogs quicker, too. Which is a bit cheeky, but that's ok now since he's good with dogs.
Our daily walk routine is working out pretty well, too. First off, we take all three for a big walk, to two parks, the one in these photos and a big forest/open area, then back home. Then we take Dusty out again while he's still tired, to this park, so that he's easier to control off the lead. We're going to work up to eventually taking him off lead with the other two, seeing what will happen.
He's suddenly matured a lot since his birthday, I'm really proud of his progress. He's not the naughty little terrier anymore! Yay! :)