I've always held my kids on my lap until they were 2, but you might need to get him a seat for your own comfort due to the belly. Keep in mind if you do buy him a seat, his car seat needs to be FAA approved, so call your airline and find out.
I traveled with my then 7 year old and 18 month old twins last summer (with a helper!). Some things I did to keep the twins entertained: reusable stickers so he can stick them on the tray and move them around w/o damaging anything, mini Magna Doodles, Crayola Color Wonder marker packs, portable DVD players, a few new small toys to get them excited once we were in the air, and LOTS of food!
They always offer me priority seating since I have kids, but I've found that it works better to board last so the kids can run around as much as possible and burn some energy before getting on the plane.
Good luck! :)