Note the station name is different for each cancel. Since the cancels are from the same zip code here, I would guess that including a note to get all 3 cancels on one envelope would work if sent to one of the stations. Make it very clear what you want and very brief. The problem is that we never know the size of each cancel so something larger than a standard #6/#6-3/4 envelope would probably be needed as a guess. Most collectors prefer small/standard size envelopes. Unaddressed envelopes are also preferred, so either use very small peelable labels to address each cover or a soft pencil that can be erased with a soft eraser.
If not using the Apollo 11 stamps, use a stamp that is light(er) in color and neutral like the flag definitive. It shows the cancel better and does not distract. Apply stamps with very clean fingers; you don't want oil from your hands causing the cancel to smear.
It is highly recommended to provide an addressed return envelope (with postage, of course). Nowadays, it is unfortunately very common for pictorial cancel covers to get overcancelled along the way.
Add a stiffener to each cover; manila folder cardboard or equivalent is good. It gets you a clean cancel, too. 3 small covers done this way plus a slightly oversize return envelope and their mailing envelope just weighs under one ounce. I send these using a #10 business envelope and a cut-down #10 for the return envelope. Of course, you have to worry about exceeding the thickness limit for just dropping in a mailbox if you send any more covers. Still, unless you are sending out dozens of covers, it's still normal first class mail; no box needed.
Lastly, your son might have to be very, very patient. Pictorial cancel covers can take up to several months to be returned, though I would expect Cape Canaveral to be more on the ball and faster than other cities.
Try to send these out as soon as possible despite the time extension given. There probably will be other pictorials for Apollo 11 so keep checking the new postal bulletins.
If he hasn't bookmarked it yet, this would be of interest to your son:http://www.rhcourtney-collector.com...duction.html