He resents the attention you pay to his siblings. Any left over energy you have for him is spent on demanding him do this and that.
The realization that soon he will be out of this house engenders apprehension. He doesn't feel ready to leave the warmth and protection of this home. He'll do anything to distract himself from the thought that he is alone out there and has to be responsible for himself.
Spend some time together with him, just you and him. Go to the places (or do things) that you and him used to go before the arrival of his siblings, even if risking look silly, "you used to be scared of the clowns at the county fair."
Remind him that he needs to be responsible for himself. He may not feel like it. But let him know that he is worthy of that effort. He may think he can get away with it. But there is a price to pay for not taking up the responsibility for oneself. He may not even know it, but the price is paid one way or another.
Be nice to your eldest child. Otherwise, dysfuntional family ensues.
