Well, I know lots of people who make them for whatever reason. This neck of the woods is highly Catholic. Aside from the ones made out of bathtubs where one can tell it was indeed once a bathtub, most people just put them together has it pleases them and the city code.
When you go to Home Depot, don’t use the words grotto, shrine or Eagle Scout. Instead, use words like brick outdoor patio grill, water feature without the water, and faux rock formation. Those are the same words you should use when you google up the how-to stuff, and when your son goes to the library to get how-to books. Also have your son go to www.diynetwork.com and www.hgtv.com and search those words. Those are the same principles and skills your son needs to apply to his grotto.
Mine is made out of cement slabs and begonia bushes.