Instead of saying "Grow somewhere else" or anything like that, tell him info that he could use for his underground construction.

If I was you, I would build a rectangle frame going around the hole with 2x4's then put a small stake or some re-bar in each corner, then lay the tarp in the hole (What about weather stripping for a sealant) and attach the tarp to the wood to create the walls and ground. Then get your plywood to fit on top of the 2x4's and attach tarp to the bottom of the plywood so you have a fully sealed area from the tarp. Make some holes where necessary and use some sealant on those if you want to really control it. I'll post a picture soon.

Personally I would rather put a steel shipping container where I live instead of doing a stealth grow in my attic but after digging 4" with a shovel I hit red clay, the thing Native Americans loved and I now hate.