He exhibits only two parts of his personality. Both are childlike, and argue about who is to be in control.
One of the psychotherapist’s roles is to act as temporary parent, unconditionally and positively regarding every part of the client’s personality. Over time, so the theory goes, the client will introject the positive regard and become that Positive Parent to himself.
Invite an ongoing dialogue between his two parts. Encourage them to argue with each other. Accelerate until somebody in the room (not you) gets sick of the arguing and refuses to do it any more. Wonder what another alternative is.
Invite two chair work. While the two parts are squabbling, interrupt and invite him to stand on the other side of the room with you. Point out that the two parts are still over there in the chairs, arguing. Ask him to wonder about the two parts, what they each want, what they would be willing to settle for, how committed each of them is to struggling, and how they got this way. This accesses a third, non-arguing, possible Observer part of him with whom you can communicate, also.
Participate in his creating a new personality part. Let’s call it Ideal Parent. Ideal Parent has as many children as the person has personality parts. Ideal Parent loves his children, approves of them being different and approves of each of them trying to get what it wants. Ideal Parent sees the positive intent, validates, and listens carefully to each of them as they struggle for control. Ideal Parent frequently verbalizes his appreciation of all. Ideal Parent also takes good care of himself, sets limits for himself, and makes the ultimate behavior decisions. He is not afraid. He is clear he is in control, therefore he can patiently allow his children the freedom to work through their rage at the fact that they do not have control. From time to time he may need a vacation from the squabbling, so he finds a quiet place in the house where no fighting is allowed and he retreats there as necessary.
Every session, pull up an empty chair for Ideal Parent. From time to time, invite the client to sit in that chair and speak from the Ideal parent persona.