As a honorably discharged veteran going to a military event, do I wear old dress greens or do I need current dress blues?

You always see WWII veterans wear their old service uniform... obviously you are not up in age with WWII veterans, but I think that point is that you wear the uniform you served in. Congrats on your son's graduation and commissioning!

To be honest, I would wear a black suit and minis on the lapel mid way down. I see it all the time at balls.

SGT Steve Oakes I found this regulation.
Section 30-1 Occasions of ceremony
Gender Specific: Both
a. As used in this regulation, the phrase “occasions of ceremony” means occasions essentially of a military character, at which the uniform is more appropriate than civilian clothing. These functions include, but are not limited to: military balls, military parades, weddings, and military funerals; memorial services, meetings and conferences; or functions of associations formed for military purposes, of which the membership is composed largely or entirely of current or honorably discharged veterans of the Armed Forces or reserve components. Authority to wear the uniform includes wear while traveling to and from the ceremony or function, provided the travel in uniform can be completed on the day of the ceremony or function.
b. All persons wearing the Army uniform will wear awards, decorations, and insignia in the same manner as prescribed in this regulation for active duty soldiers. For civilian attire, individuals may wear only those awards, decorations, or insignia authorized by this regulation for wear on civilian clothing, in the same manner and approximate location as the equivalent military uniform.

http://ar670.com/chapters/view/30