Hello,
I have traveled with our son (6 years old), our daughter (11 months old), my neice (was 2 at the time), and nephew (6 years old). You might be surprised at how much more comfortable it is to just hold your youngest on your lap. You also would save some money, because you would not have to pay for an additional seat for him (at least you shouldn't since he is 2 years old). I know that it seems like a pain to have to hold your child for the duration of the flight, but really it isn't all that bad.
When I flew with our duaghter, she was only 9 months old at the time. She slept through most of the flight and when she wasn't sleeping, she sat there and played with her big brother or one of the small toys that she loves to play with. On the way back, I had to fly without my husband due to some rental car issues that we had no control over. I thought it would be challenging, but she was a complete angel. We chose to hold her to save on the cost of an addition seat for her car seat. We just had to check the carseat with our luggage.
Just keep in mind that you don't have to have a child restraint on the airplane. Personally, I think they are a hastle (you have to carry it on, strap it in, and then strap your child in it, all the while keeping your child from running off or goofing around) and (as morbid as this is going to sound) they aren't going to save a child's life if the plane were top crash. Sorry if that sounded harsh!