K.B.
A couple thoughts... My neighbors park their car sideways in their driveway in front so the kids won't roll out into the street on their little cars, etc.
Another neighbor puts a Sign out front that says Child at play, it is on a workhorse stand, she bought it somewhere... and sets it in the street out front of her house.
If you can't put a little fence in your front yard, could you get some chicken wire to post out front when you are out with him?
As for concrete.... you could get him some knee pads ( lol ) or maybe go get some cheap carpet to lay out there for him to play on?