Name: tarev San Joaquin County, CA (Zone 9b) Give PEACE a chance!
That maybe Sempervivum arachnoideum, it forms those cobweb-like fine white threads. Not a cactus, it has no areoles that has thorny spines.
This is my plant when I first had it in March 2013: they are actually cute, if they get to multiply and form a mound. Your son might find it interesting to pull the new babies as it forms though.
Editing to add this other photo of my other semp arachnoideum before, more tight, mounding type growth, it came from a very small container before, so it looks less flat , but it still has the cobweb-like hair at the center of each rosette:
