You have gotten soo much great advice I just wanted to repeat the part about preparing your child. I thought I had when I took my twins on their first flight and thankfully I don't think this is an issue anymore because I don't think they make children do this anymore but.....we had complete meltdown when my son (age 4) had to take his shoes off to go through security. Seriously, I think the poor thing thought he would never see his shoes again, it was very bad. I thought I had prepared them for the experience but the shoe thing had just started when we were traveling and I had never experienced it either so I didn't know. The way home was a breeze because he knew what to expect, soooo much of their world is about expectations and I totally know that, am usually great at it but I got a F for failure in a big way for the shoe thing.
We flew Jet Blue for the sole intention of the televisions. Like others have mentioned, we didn't do much screen time at that age so being able to sit for 3+ hours with a screen was total bliss. The flight was a breeze. My son really loved watching the channel where he could see the direction the plane was going and the speed/altitude etc. One time he was soo excited he yelled out "mommy, we're going the right way" and literally it felt like the entire plane laughed out loud. Once we were able to unbuckle and start to leave the plane I could see people trying to get a glimpse at him, you know the kid who was soo excited to be going the right away. Anyway, the screen was all we needed for the entire flight for my children because they were soo enthralled with not having any interruptions for hours of straight viewing. I had plenty of other things to pass the time but never needed anything (except the lollipops and covered water bottles to help with ears).
This is so kid dependent. We have flown when our kids were 3.5 yrs and 15mo, 4.5yr and 2yr, and 5.5yr and 3yr. We have never prepared them beyond "we are going to Florida on an airplane."
Caution about loading them up with snacks. When our youngest was 2yr, she got sick on the descent. Luckily, she was rearfacing in her carseat so it was contained to her lap and not all over the floor, seat back/pocket etc.
If you bring a water bottle, avoid ones with straws. The pressure build up causes them to spray water when you open.
Another vote for the CARES, the straw cup for take off and landings, some snacks (maybe special ones he doesn't normally get) and carrying on a change of clothes for him just in case. If you have an iPad they can use, download a movie that you think they will love as well as a few episodes of shows that he will like. A headphone specifically for kids is best (over ears instead of in ear). I always found it was good to have little activities for my son in case he didn't want to watch the iPad. A few suggestions... wiki sticks, one of those characters that's in a mystery packet (for whatever he is into), triangular crayons (they won't roll off the tray), and a small amount of mad matter or morph. Those don't take up space and you never know when there is a delay as well. We flew a lot when he was little so I kept those only for flights so he was always excited to play them. He's 6 now and he still wants a new mystery packet when we fly. I practice the "be prepared" method.