Assuming you're being facetious, but I have the feeling if his father heard the blades being engaged, the boy would have more issues to worry about than the mower LOL
Couple months ago we had a community event inside the school gym with those large blow up dohicky's the kids jump and slide on. Some boys we're getting out of hand running around the gym with a basketball, and sooner or later someone was going to get hurt, and I told those boys twice to lay off the rough housing with the ball. They didn't. I went up behind the boy will the ball and grabbed it from them and said they lost the basketball. Man, they made a scene being upset on what I did, and honestly I felt like I was kind of in the wrong because these weren't my kids, and I really wasn't nice about taking the ball away, and I wasn't giving it back to them. Honestly, flash back to cub scouts when kids were getting out of hand and I had to put them in their place because they wouldn't listen to me (my one boy included). I got a couple of parents upset because I raised my voice to the kids...
At the gym, the boys mother (boy who was riding the mower) came up to me after I took the ball away and I was apologizing for taking the ball away. She looked at me and said no need to apologize, and what I did was great because she saw them starting to get out of hand and would have done the exact same thing I did (she was busy watching 3-5 year olds), and she was glad I did it.
When my boys were growing up, if they ever got out of hand, any adult had the right to discipline them if the adult felt like it was warranted and I wasn't around. NOT all parents feel that way.
Kids will be kids, but sooner or later with no rules set in place, sooner or later things are going to get out of hand. Saw a kid get hurt and taken to the hospital because 10 of them were rough housing on a see saw of all things, and not one parent (out of a crapload around) said a darn thing and I kept my mouth shut even though I knew it was an accident waiting to happen.
My wife and I are kind of the "old couple" with no kids at the house anymore from church, so I'm on my toes when I think I have to correct the behavior of kids who aren't mine LOL