D.T.
When my kids were barely 3 yrs and a 3 month old we flew from Denver to Palm Springs and it was fine. Talk to the airlines about any special requirements, they should be able advise you about specific car seats and drinks. If you don't want your son drinking juices that have sweetener added you may be out of luck. At that age they shouldn't be drinking undiluted juices anyway. How about apple slices? Put them in a ziplock bag along with a little "fruit fresh" or a crushed vitC disolved in a little water and they won't turn brown. You could also prepare little baggies of grapes, melons, banana slices, cheerios, crackers. It's only a couple of hours - you don't need much. Bring things to keep him busy, have him help prepare his bag with a couple of books, a paper pad to color, a ziplock with some crayons. I made my kids some little hand puppets as a surpise for traveling.
There are some good books about traveling to help prepare kids to know what to expect and to build some excitement about it. Visit the library. Kids take their cues from you, if you are calm and positive, generally they will be too. If you are nervous or ambivalent they will pick up the feeling that something is wrong no matter what you say. Relax and enjoy yourselves, you'll have fun! It's an adventure.