One of the things used to pack wounds is an expanding foam - squirt it into the hole and it expands to fit. As the hole heals from the bottom up, the foam plug can be replaced with ever smaller ones. The wound must heal from the bottom up or there will be a healed over hole with a reservoir of infection inside, and no way to drain. I'm sure you know that. A similar expanding foam is used in the building trade to fill holes which are not a regular shape.
The alginate dressings are seaweed, but I don't think they are used so much now. I always thought seaweed was great, but it does nip a bit, and has to be redressed frequently.
It does look as if the horn is healing well, but I see what you mean about the hole filling up with gubbins and having to be flushed out, which can be done with a large syringe, no needle, of normal saline, then suck out any fluid left at the end of the process.
Well done for how he's recovering.