As I remember, the Knight BK 92 was very similar to the MK 85. Doesn't that have a #11 percussion cap ignition? If it does, then load 80 grains of loosePyrodex RS and a 240 or 300 grain XTP. That would be a good starting point. Also with that kind of a charge the young man should be able to handle the rifle just fine. Another thing you can try are 300 gr Saber Tooth conicals on 80 grains of powder. Again, an great load and should do all you need it to do. The rest would boil down to practice.

So check the rifle breech plug and see what kind of ignition system you have. Then we can suggest perhaps all the things you might need to make the rifle work. Also read the manual, there is a ton of information in there for you as well.