Not accepting No

Hi,

My son (14) has learnt to take advantage of situations where he feels he will have the upper hand and knows when we really need his cooperation. We feel like even if we say no, we still must give in to his demands or he will not cooperate with what we need him to do. For example If my son doesnt get the candy he wants at an airport gift shop, he will refuse to move from the store or he will start walking in the opposite direction we are going. In normal circimstances, I know we shouldnt give in or let him push our buttons but in a situation like this, we must have his cooperation or
we risk losing him and missing a flight.
Even if we talk about this in advance, when we are in the actual situation, he doesnt always keep his word.

Another example would be that he requests screen time first thing in the morning as a condition for him to get ready for school. If I were to tell him that he needs to get dressed first and then screen time, he would probably threaten that hes not going to school..How can we handle this so that he accepts a no answer and cooperates with us even if he doesnt get what he wants (so we don't miss a flight)?