Frankly, it's ludicrous that he thinks you need to give him "proof" that the diagnosis needs to be specifically stated on the OTV note. Of course it does! (The physician doesn't need to assign the actual code, but they at least need to state it in words. It's not for the coder to guess whether the treatment site is primary or secondary. For example, the physician doesn't need to assign C79.51, but the documentation should certainly state that it is a Secondary Malignant Neoplasm of Bone.)
The OTV is like any other office visit note and needs to be able to stand alone. If an auditor or an attorney or another clinician were looking at that document, they should be able to tell just by looking at that note what the patient's diagnosis was.
I'm not sure leaving it off would really save much time anyhow, honestly. If he's using ARIA to document the OTV note, the template should already have a space for the diagnosis to be inserted. (I don't work with Mosaiq, but I have to assume that it's similar, as with pretty much any EHR.)
I'm having trouble getting into my ASTRO eBook opened today, or else I'd search the coding resource to find a specific ASTRO reference.