Definitely doesn't look like scale rot, either dehydrated skin or stuck shed. Raise the humidity to keep the boa hydrated and to ensure a good upcoming shed, since you did mention he's got signs of pre-shed.

Like the above person said, Rainbow boas need high humidity to thrive. Are you keeping track of humidity levels in his environment? If so, what are they? Do you have a humid box for him with something like sphagnum moss to keep humidity constantly high?