entitlement for a member of the office staff?
From September the new staff governor has to be elected by and from all the non-teaching staff, so what you suggest isn't possible.
Your schoolkeeper can choose to serve the remainder of his term as a co-optee, although your support staff may decide of their own accord to vote for someone in a contrasting role. But it is their choice, which you must not try to influence.
You won't be able to co-opt any member of the staff, teaching or non-teaching, after September. So your schoolkeeper has to decide whether he wants to stand for election or finish his term as a co-optee. If the former, he has to take a chance on getting elected and having a full period of office ahead.
But if he finishes his term as a co-optee he will have to wait until the next round before he can stand for election.