I have flown a lot with kids from 5 weeks to 5 years, and several times with 3 in those age ranges. Do not take a car seat. I have regretted it every time, except for the time it was late night on a puddle jumper, and then only for safety, and not comfort or ease. Your car seat will state on it if it is approved or not. They will check it at the gate.
I would strongly advise against a late night trip. Our kids have never slept on those and it's a miserable trip. In fact, one time, on a 5.5 hr trip, my, then 3.5 yo, cried the whole time while the 6 mo old also cried. When the captain announced landing, they both went to sleep. It was after 1 a.m. when we landed, and I had to carry the kids and all their stuff off the plane because they were then too tired to walk (the older one).
I would advise taking a regular size stroller so you can load the basket with things and not have to carry everything and push. It will make security a big hassle, but the rest of your trip will be easier, if your son will ride in it. I recommend that for toddlers, but not babies. You can wear a baby through security. I doubt your son would go for the Baby Bjorn again, even if you could manage it. :)
For entertainment, I take a new coloring book and toy for each way. One that has been put away for a while works just as well if you don't want to spend money on that. I've never used the DVD. I like to go simple. Have your son help pick out toys and he can carry them in his own little bag. An extra pair of pants isn't a bad idea to have in there either. I have known little boys to not like the airplane bathrooms. :)
Have fun!