The fact that they haven't thrived could be due to the lack of vitamins. You can dust anything they eat, "shake and bake" style. Throw it in a bag with a pinch of vitamins and shake and dump.

A quote from the care sheet that I linked you to -
" They should receive a diet of dusted/supplemented fruit flies (Drosophila) and pinhead crickets 5-6 times a week. It is important to remember to dust both of these food items with calcium and vitamins several times a week to ensure an adequate growth rate."
I would skip the blood worms and just do the fruit flies or pinhead crickets if he's not big enough for anything else, which seems really unusual at 6 months. What is his length snout-vent? Can you take a pic of him with a penny by him?