I think it might be wiser to wait on the supp in order to get a true baseline reading of the ACTH without interference, no?
The way to "catch PPID early" is to start documenting daily any unusual behavior, attitudes, lameness, changes in eating/drinking, etc. Take pictures of body condition (all four sides) every couple of months to compare objectively. My mare has completely atypical appearance for PPID but is in fact early stage. She had enough little odd-ball symptoms cumulatively (plus refractory insulinemia/hyperglycemia) that warranted a pergolide trial last fall. She responded very well and her IR is now much easier to control. She still does not look anything like a "Cushing's horse" now almost a year later after diagnosis. In fact, she probably looks the best she's ever looked, now, at age 21.
Do you have any reason to believe he is insulin resistant?
Kerry in NY