Since your child is over age 2 he has to be in his own seat. At 2 it's much more comfortable for him to sit in his own car seat. Depending on the model, you can make it easier to carry the car seat (and the kid) using an attachment device that will turn the seat into a piece of rolling luggage
We use the original GoGo Kidz for our son's Britax and it's great! No need to carry that heavy thing or walk around with strapped on looking like a turtle!
There is also a chair that attaches to your rollaboard
The newly FAA-approved straps are okay for a 2 year old but best for older kids who can see out the window.
*NOTE* ANY child in a car seat MUST be seated at the window!! FAA rules. The only time that this wouldn't be enforced is in the case of a widebody aircraft (two aisles) where mom/dad and the child take up the entire center section of the row (ie mom, kid in car seat, older sister).
The car seat is for the child's comfort and safety just like in a car. Yes, during a catastrophic event, the seat will provide no more protection from severe or fatal injuries like the regular airplane seat for the adults. However, the child seat WILL provide better restraint for the child during turbulance, wind shear and minor mishaps. This is why it's always best that an infant (child under 2 in airline terms) who could travel for free if held in mom or dad's lap instead be placed in his/her car seat. Yeah, that means paying for the seat (maybe a 50% discount on some airlines) but in the event of wind shear, the aircraft would drop some 3,000 feet just like that and while everyone is being jostled about in their seats, the babe in arms now becomes a projectile flying across the cabin at 60mph...you do the math.
One last note....when flying with a car seat, FAA rules REQUIRE all *US-owned* airlines to allow the use of FAA (or similar international agency) approved child restraints on the aircraft...even if flying internationally. These rules DO NOT always apply to non-US airlines. Aer Lingus (the Irish airline) in fact FORBIDS the use of car seats! Other airlines are somewhere in between--allowed in coach but not in business class because the have the "pod" seats that fold into beds. But, when flying on a US airline, so long as your seat has the FAA sticker (be sure to locate it in case you're asked) then you're good to go.
PS If you want a low cost, light weight alternative, consider the Cosco Scenera car seat from Wal-Mart. Other places carry it but the Wal-Mart units are packaged in a clear plastic carrying case. It costs under $45, rear or forward facing, goes up to 40 lbs and has a 5-point internal harness. Many frequent fliers use it as their "airplane" or traveling car seat rather than having to take out the one in their own car. Plus, it's a better, safer alternative to relying on the rental car comapny's rented seat which may have been subjected to less than ideal use...