We've done 3 trips with 3 month olds (all to WDW too) and all of them were very easy flights. Actually, now that I think about it, I also took both of my kids alone to Phoenix when my son was 2 months old as well. So if that could be a good experience, you should have no worries!
My best advice would be to buy a seat and bring your son's carseat on the plane. Having a place to put our kids (even when it was just our daughter) was invaluable, plus they're safer than when on your lap. I think bottlefeeding makes it easier cause you can feed the baby in the carseat if you have to (takeoff, landing, turbulence, eating something yourself, whatever).
(I breastfeed, but try to pump a bottle when we fly cause it just seems to make life easier-so no comments about how easy nursing is on a plane or whatever!) With my kids at that age, I generally feed them right before takeoff, strap them in their carseats where they fall asleep till the next feeding.
Be warned that not all planes have changing tables so be prepared to change the baby on your lap or your seat.
Overall I think it's a really easy age to fly with a baby. They still sleep a lot and are pretty happy with just eating and sleeping and playing with Mommy/Daddy so you don't have to cart a lot of toys on board.