You also need to be supplementing with plain calcium (without D3) every feeding. D3 helps your cham utilize the calcium that is supposed to be dusted on your feeders. I would rank this as a highly important next step. While this may not be the cause of the spots, it should be taken seriously as inadequate calcium can cause mbd.
I know that too much D3 can essentially take calcium from the bones and redeposit them to other parts of the body. I have no idea if this would happen if a cham was given a normal dose of D3 without the frequent plain calcium (w/o D3) dusting. Though I doubt that this calcium redeposit would appear on the skin. I think only
@kinyonga would know that.
What is your basking temperature? And have you had him for three years or did you recently get him?