You may be the gatekeeper, but your boss is the one with the key to solving this problem. His former associates followed him in anticipation of continuing the open and communicative working relationship that they used to have with him. However, in the present company, he changed the rules of the game.
Since he now requires more time behind closed doors, he needs to meet with his staff and explain the situation to them. Certainly there will be times during the day when he can be more accessible, and his employees need to know how he wants to proceed. If he fails to communicate his expectations to the staff, he has put you in a no-win situation: either you are a nag who keeps the troops out, or you are a weakling who lets them walk over you.
The larger issue is why your boss is reverting to a less accessible managerial style in the first place, particularly in light of the fact that effective management today calls for a good deal of contact and communication with the staff. Perhaps this issue will come out in his discussion with the troops, or maybe it could even be brought up by a highly insightful executive secretary.
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