K.Z.
I have flown with my daughter twice, once when she was 4 months and then when she was 10 months (it was just the 2 of us at 10 months). The pressure changes didn't seem to bother her at all. When she was 4 months old I nursed her during take off and she slept the rest of the way. I didn't nurse my 10 month old on the plane, but she played and slept nicely.
You can check the carseat either when you check-in or at the gate (same for the stroller). A front carrier was the perfect thing to get around the airport - even better than the stroller.
My advice is to check everything you can as early as you can and carry just what you need for the flight. Since your little one is still mostly nursing, he really shouldn't need a lot of stuff. A pacifier, blanket, a few small toys, and his diaper stuff. I put my purse in the diaper bag so I would have fewer things to carry. I wore jeans so I could put my driver's license, cash, and a credit card in my pocket. That's all you need for security and it makes it easier to buy some water or a cup of coffee along the way.
I was stressed too, but it was easy and fun both times. Good luck!