You can't script this type of thing very well, and someone is bound to get hurt.
As the mother of a 13-year old (who also likes to sneak up on his step-dad and wrestle around like a maniac), I fully agree with Opher's techniques. Hug-nage is most likely what the squirt will respond best to (after the initial struggle).
Actually, I just asked my son how he would like this to be handled (putting him in the shoes of the other 13-year old), and he said that the step-dad should just tell him firmly but kindly to stop.
Hope that helps,