We took a 2 hour flight to
Disneyland when our dd was 4 months old. I was worried too but it turned out to be very easy...much easier than flying with her five months later!
For the pressure, feed him when you are taking off or give him a pacifier if he takes one. We had a bottle prepared for our dd but she fell asleep due to the engines running on the plane. We tried to wake her up to give her the bottle but she wouldn't wake up (a first and last for her) so we just let her sleep and were scared the pressure would get to her. It didn't and in fact, she slept until we started making our descent.
She took a little bottle when we were coming down but not much. Coming home was an almost identical experience except she only slept for about an hour.
I highly recommend bringing the car seat and paying for a seat. Check with your airline too though. With the two airlines I've flown with with dd, you can opt to not buy a seat and they will let you put your carseat in a seat if the flight isn't full. If it is full, they will tell you when you check your luggage and you can just check it then.
We didn't want to risk it though and just bought her a ticket. It was much easier and one less thing to stress about.
Carseats are pretty big and cumbersome. I've seen some parents with straps for them so they can wear them like backpacks through the airport. We brought our stroller which the carseat snapped into. This was nice for pushing her through the terminal. We were also able to hang and store our carry-ons on it too. We then left the stroller with them just before we boarded and carried her and her seat onto the plane.
Another tip is to try to keep your ds awake at the airport as best as you can so he will be tired when you board the plane. I got the idea from a baby I saw several years ago just before a 5-hour flight. I couldn't understand why the parents wouldn't let this kid sleep. He was obviously very tired and irritated and fussing a lot in the terminal. I was just hoping I wouldn't have to sit near them but I did and he didn't make a peep the entire flight because he was so tired!
Also, be aware of the security laws now regarding flying with liquids. Most baby stuff is accepted now but it is limited to quantities and how much the container can hold. I don't recall if you can bring pre-made or pre-pumped bottles. Be sure to check with your airline or TSA before you leave. We bought ready-made cans of Nestle's Good Start. You can also bring powder and buy a bottle of water when you get past security. You won't be able to bring bottled water through security. Again, check with your airline or TSA as it's been a while since I flew with her and don't recall the laws.
That's all I can think of for now. I hope that helps!
-steve