D.P.
Sounds like you are flying Southwest. My biggest piece of advice is to make sure you check in online 24 hours before you leave. You NEED to get in the "A" group. Trust me. Southwest changed their boarding policy back in November and people with kids under 4 can still pre-board, but only after all the other pre-boarders and the A-group. If you get a boarding card that says A-group you get to board with the A-group; you will be closer to the front (less stuff, seats and people to walk through) and there are fewer people saving seats for their friends/family in the C-group.
I did not bring my pump because I was going to be around my son the whole trip to breast feed. I did bring my sling for breastfeeding in public/on the plane. Worked great!
I didn't have any trouble with baby food/milk. I packed a small cooler with his food. (Much of it was frozen ice cubes) because I make my own) and I bought water on the other side of security for his oatmeal.
Traveling alone, I limited the number of carry ons I had. I have a backpack that he can sit in, so I brought that and one other bag + his stroller. I gate checked his stroller. When we went down the ramp to the plane, I put him in his backpack, so I could easily fold the stroller up with 2 hands. (On the way home he was in the sling, which worked great too.) I put the backpack in the overhead compartment when we got on the plane, and he slept in the sling most of the way to Florida.
Southwest has covers for their drinks! Ask for a cover for your drink (and his if he gets juice). You don't have to worry as much about a squirming baby and bumping the tray.
As for the bathroom, I took him with me in the sling. A little (ok, a lot) tight, but do-able. On the way home, I went before I got on the plane and held it until I got off.
Good Luck,
Danielle