T.H.
Well I vowed never to be this mom, but I am, so....get a DVD player! ;) It is seriously a life saver. Other than that I would recommend books, a magna doodle, maybe some stickers or crayons. Color Wonder markers are also really good to use on airplanes. I would also take a ton of snacks, new fun things that he will enjoy to help pass the time. Maybe some card games also. My daughter just turned 3 and she loves to play go fish and crazy eights. A few rounds of cards with her is easily 30 minutes, so pick up a couple decks of those. Those would also be good if you have to be quiet inteh house with grandpa, etc.
I like flying SW, I think you'll have a good experience, but the only issue is they don't let families board first. You have to get an A pass to board with the As so get on to print your boarding passes EXACTLY 24 hours before your flight. If you don't end up getting an A you can board after As and before Bs which isn't terrible, but it certainly isn't ideal either! Good luck!