I just now noticed that I suggested gutting a teddy bear in two different posts. Apparently I have unresolved issues. Sorry.
We forgive you, seawitch. 
I think you should mail all of the rocks to him COD.
More on this option from one of my favorite magazines The Journal of Improbable Research.
http://www.improbable.com/airchives/paperair/volume6/v6i4/postal-6-4.html
There they discuss all the improbable things they tried to mail and the results. The conclusion I came to is that the USPS certainly puts up with a lot. Makes me proud to be an American.
I have one suggestion if it’s a small rock-- inside the toilet paper roll. Between the spring thingie and the cardboard inner…
Does he use a computer? Does he have an optical mouse? If not, you can see where this is going…
Take out his mouse’s trackball, and replace it with a fairly (but not quite) round rock, just a little smaller than the ball so it will still work somewhat. When his mouse starts going crazy, he’ll turn it over and find - the rock! Keep his mouse’s ball around though, because he’ll want it back.
Order a porn video. At the point where things start getting, uh, “heavy,” record over it so that the rest of the tape is a shot of a rock; just sitting there. Then put it back in the box, replace the cellophane (seran wrap and a blowdryer work well), and mail it to him. He’ll have had to put up with the horrors of a porno exposition, then is teased into the joke - a rock where nobody expected.
As long as it’s big enough not to get sucked past the U-bend, in the bottom of the toilet bowl and then put the lid down.
Does he drink coffee? In his cofee cup (upside down).
Hollow out a tomato and insert rock into hollow.
Slight hijack: This makes me think of a news story I read some years ago. A man (I think in Florida) every year on Christmas would get a coconut delivered to him. Attached to the coconut was always the same message: “To the Daddy from the Thingy.” He had no idea who kept sending a coconut to him, or why the person would go to such great lengths. For example, one year the coconut was delivered by helicopter. Another year it was brought to his front door by a semi-pro basketball team.
This is funny if it happens to someone else. If it happened to me, it would freak me out severely.
Oh, that coconut thing is funny! (Creepy-funny)
These are all great ideas! Loved your stories, Ich Bin and Telemark! Hee.
I love the mouse idea but these rocks are bigger than that–they’re at least two inches if not more.
Get a Gideons copy of the Bible, open it to Lamemtations and cut out a space for and deposit a sedimentary rock. Leave it on his bedside table.
A can of ground coffee.
A large box of laundry detergent.
Local Pizza place.
Go there.
Give rocks & Instructions.
Pay for delivery & give a big tip.
It’ll smell so good at the door…
“Yes sir. I have a large Pepperoni with EVERYTHING on it, pre-paid.”
Wanna bet he takes the rocks off and eats it anyway.
Oh, he would SO take off the rocks and eat it!