This answer may not be totally satisfactory because of the limitations in the definitions for Chronic Kidney Disease.
Stage 1, as you know, is poorly defined as:
a) microalbuminuria or proteinuria, or
b) structural disease by imaging, such as ultrasound.
You haven't given me the ultrasound reports because the patient hasn't had one. Thus, with the information you've provided, he does *not* have CKD stage 1. Though this answer is practically laughable because after 20 years of diabetes, one can assume with high certainty that there is probably some damage that has occurred (though not a tremendous amount and perhaps still fully reversible).
Below is a chart that quickly explains the definitions of CKD as per K/DOQI guidelines.
Now, regarding an ACEi or ARB: strictly speaking, this patient does not require such a medication, especially since he is not hypertensive (another indication that the kidney dysfunction is not severe or permanent or even clinically significant). Having written this, however, I would consider starting some form of RAAS blockade because of the 20-year history of diabetes. There is no evidence to support this statement (at least none that I am aware of). Even the AASK trial does not provide an answer to this question.
I hope this helps somewhat.