he's retired and can do as he pleases.
No, he cannot.
Uniform Rules for Veterans and Retirees
The rules for wearing military uniforms as a retired military member or a discharged veteran are similar for all the services. There are certain rules for those seeking to wear the uniform for formal functions, national holidays, parades, military funerals and weddings and other military occasions. Only the Service Dress Uniform may be worn; no work, battle dress or PT uniforms are permitted to be worn at formal events. For non-formal events, veterans are allowed to wear other working uniforms as deemed appropriate for the occasion.
Grooming standards are obviously not enforced when a veteran wears any of the uniforms, but it is common courtesy to wear the uniform as if you were still in the military conforming to all the hair, facial hair, finger nail, and other grooming standards of the military branch you represent.
All veterans and retired members will conform with the same standards of appearance, military customs, practices, and conduct in uniform prescribed for active-duty.