He looks so cute! I'm glad to hear he is doing well!
I agree that wild house flies may not be a great feeder since you don't know where they've been.They aren't a common feeder so it might be a question for a experienced vet/specialist exactly what the risks are. You could look into breeding them yourself in an enclosure, I can't imagine it's hard, but probably gross. Have you tried black soldier fly larvae? (Phoenix worms). They are grubby like the butters but much more nutritious. As grubs they are a good source of calcium. They pupate into flies that some dragons love to eat as well (not high calcium though). If you are in the south us you can even culture them.
Tonja, where is that bit about the diaphragm from?