I would take a few potty pads so you can keep the bottom of the travel carrier lined in case of accidents. To give him a break from having him in the carrier under the seat, take him and a potty pad to the rest room and let him walk around for a bit of exercise. Give him about five minutes anyway.
Also put a little sweater on him because it can get pretty chilly on those flights, especially by the floor. Take along something he can chew on, but not eat, and maybe a tiny toy. What I did with Zada when she was a little puppy was put a blanket over my shoulders and front, and put her in my lap. No one could see her and she slept so good.
(I'd take the sweater off while he's sleeping on your lap though).