I agree with the PPs -- I've never lugged a Frontier through an airport but it looks like it'd be awfully bulky. And at 19" wide, to get it through an aircraft aisle you will probably have to be lifting it pretty high.
How big is your 6.5yo? If he's big enough and mature enough, I'd consider bringing a booster for him or having one waiting at the other end -- even if he's normally harnessed at home. My older kids love their harnesses but they understand that sometimes for ease of travel they need to be in boosters. If you're not comfortable having him in a booster yet, is he still small enough to fit in a Boulevard? Because if he is, it'd be easier to bring your Boulevard on board for your 2.5yo and have the other waiting when you arrive.
Bringing the Frontier would technically probably work, but changing strap settings etc. before departure and again at your destination also tends to swallow up some time.