Section 4C. Whenever there is located, wholly or partially, within a planning district established hereunder, a federal military installation having a resident population of at least five hundred persons according to the most recent available federal decennial census, the district planning commission may vote to offer to the commanding officer of said federal military installation the privilege of membership for himself or his designee. Upon acceptance by the commanding officer of this offer, he shall be deemed a member ex officio of the district planning commission, and shall assume the same rights and duties as other commission members, except that this ex officio membership shall not continue except by an annual affirmative vote of the majority of the commission.