rockm

You have a few issues that you will have to overcome using boxwood as root over rock.

The first, and probably most problematic, is box's habitof pushing shallow, dense, fine roots. They do not push thick primary roots like tridents. That makes achieving the long primary roots necessary to drape over the rock and into the soil extremely difficult to get.

The second issue here is the initial choice of material. A root over rock tree usually has a trunk that is similar to a cascade--not a bolt upright trunk like this one. A trunk with more lateral movement complements the rock. Long tall trunks de-emphasize the rock.

Thirdly, the plant here appears rather weak and the treatment you're giving it now will probably weaken it further. I would move the tree (along with the rock into the ground and out of the container. That could give it a bit more breathing room in the health department.