J.N.
Hi H.,
I think a carseat can help keep a busy body confined to his seat as the airline seatbelts are very easy to unbuckle and it can be a challenge to constantly keep a little one buckled in--especially since they see others walking around they want to do that too.
A carseat can be a bit bulky and difficult to take around with you, however, at the age of two it is worthwhile.
If your son was 3 or 4, he would have a better understanding that he needs to stay in his seat while on the airplane, but I have found that at the age of two, a carseat is a bit easier to keep them secure--and bring lots of new activities to keep him busy such as a portable DVD is a lifesaver on a plane.
Good luck!