Dust his food only once per week, and cut the amount by at least half. Mineral supplements can cause crystalline body inclusions and huge bladder stones. He gets all he needs from his diet anyway.
The basking temperature should be about 100F. Then let him choose where he wants to bask. He will use the 100F spot, and also use the places that are not directly under the heat lamp. I've rescued iguanas in the past and they all came to like 100 to 105F heat, and 90-95F body temperature directly measured (with a heat gun) body heat. He won't stay in the high heat very long but I think they need it.
Heat guns measure the temperature of what you aim it at. Your ig will often like it when his skin registers 90-95F.
Your ig is going through what my igs used to get. Often it was from parasitic bacteria and organisms. They get a tummy ache and stop eating. Bring a fecal sample to your vet if you can get one. Hopefully your vet will do an x-ray, a blood test and a fecal exam. The blood test can spot dehydration among many other things.