Ralph is stressed since all the boys are relying on him so he is relived when Jack wants to take the lead for a moment because this means Ralph can relax a little bit. This is foreshadowing to when Jack completely takes control and begins his own reign putting everyone's lives at risk including Ralph's.
This tells me that Ralph has a lot of weight on his back. Everyone depends on him which makes him stressed and not want to deal with anything. He doesn't want to be the leader. This moment foreshadow's that something is going to come up and Ralph needs to take the leader position.
