He should tell the MEPS doctor everything he has to tell--provide any supporting documents. Not sure what good it would be to tell the doctors there is more infomation to provide but then failing to provide it. If you believe the additional information will help your son in some way--tell the recruiter he can't make the Monday MEPS call. Understand the desire to be responsive to the recruiter, but if its better to wait, tell the recruiter and reschedule the MEPS appearance.
If he does not reveal a condition and later the government discovers a condition that pre-existed his enlistment, and the government believes he knew about it and concealed the information--he would very likely be discharged--maybe under a fraudlent enlistment charge.
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Army Regulations set out the rules all the military services follow for disqualifying and non-disqualifying medical conditions at enlistment or which happen during service.
Psoriasis can be a disqualifying condition.
"2–28. Skin and cellular tissues.
o. Current or history of psoriasis (696.1) does not meet the standard."
Therefore your son should disclose this at MEPS. During that time there is the possibility that they will give him a waiver to enlist. But he should be prepared to be rejected for the medical condition.
1–6. Review authorities and waivers
a. Medical fitness standards cannot be waived by medical examiners or by the examinee.
b. Examinees initially reported as medically unacceptable by reason of medical unfitness when the medical fitness
standards in chapter 2, 3, 4, or 5 apply, may request a waiver of the medical fitness standards in accordance with the
basic administrative directive governing the personnel action. Upon such request, the designated administrative authority or his or her designees for the purpose may grant such a waiver in accordance with current directives. The Office of
the Surgeon General provides guidance when necessary to the review and waiver authorities on the interpretation of the
medical standards and appropriateness of medical waivers. The Secretary of the Army is the waiver authority for
accession. That authority is delegated down through the Deputy Chief of Staff, G–1 to the authorities listed in
paragraphs c through i, below.
If your son does not disclose and the problem is (will likely be discovered) then he has a potential fraudulent enlistment charge. That charge could result in discharge with an adverse characterization.