There is a nutritional difference between Black soldier fly larvae and the flies themselves. The flies don't have as much calcium. I use them only for entertainment value.Can you please site where you've seen/been told this? My Jacksons is picky as well but the one food source I see him gravitate to consistantly is BSF. I try to feed BSF as a treat but would love to be wrong here.
You can give exactly the right amount of calcium but with out D3 it will not be absorbed from the gut or deposited in the bones or available for use in the muscles including the heart. If you haven't been using D3 and you think your light might have been in the wrong place I would look in this direction. You will probably need vet care to get his strength and coordination back.