That’s a Dad rolling and rubbing himself on an old, smelly tennis ball — most likely covered with grimy dog saliva. Isn’t that what tennis balls are for?
When the kid comes by to check it out, Dad doesn’t let him. Dad wants the ball for himself — at least until he finishes his rubdown. When he’s through, he abandons the ball, allowing the kid a turn.
But the kid was only interested in it while Dad had it. He follows Dad, giving the ball only a short sniff as he passes it. Maybe he’ll try rolling on it another time.