How much does he weigh? Do you trust that he would sit up properly in a booster seat? If you are taking a shuttle from the airport, they probably won't have any seatbelts for either carseat but if you are getting a rental car and plan to do some other driving, I would want my own carseat that I trusted especially on those crazy freeways. He will be safer on the plane in his carseat but then you have to weigh that against the pain of lugging all that stuff plus reinstalling it once back home after a long trip.
Besides being safer in the carseat on the airplane, it sets the tone for the whole plane trip. The kids know they are to be seated and yes, you are in a moving vehicle! Most of the airline trays would fold down perfectly across the carseat and they could color and eat but we did have a fold up tray to hook on the seat just in case too. (One other tip is to take their shoes off so they don't kick the seat in front of them.) Mine were perfectly happy in their own seats and slept a good part of any flight.
We always brought our own stroller and carseat and I was able to sit the carseat right in the stroller and bungee it and then the kid could sit in the carseat inside the stroller through the airport. I would gate check the stroller keeping it with us until the last second just in case the flight was delayed. Most airlines will let you preboard to install the carseat. If they don't make the announcement, you can always ask the gate attendant. When preboarding, don't forget to get yourselves pillows and blankets on the way to your seat. Not all airlines have enough to go around.
I did not want to be sitting in the airport for hours with no stroller and I did not want to make the walk from
Disneyland to any hotel late at night without a stroller. If you rent one, you have to leave it at the security gates, can't even shop DTD! Nights when the kids were asleep in the stroller and DH and I just walked around and ate and shopped were so so nice! Once my kids were older, we did just rent strollers at DL but more than once I had to carry the little one back and DH carried the big one-sheer torture.
When I had to do TWO carseats and two strollers by myself-ugh, I bought a simple folding up luggage cart thing at kmart for around $20. It looks something like this:
http://www.officedepot.com/ddSKU.do...-_-Shopping Comp-_-Datafeed-_-Office Supplies
It would fold up small enough to put in the diaper bag and I still use those things now to carry stuff all around to the beach and to school. Then, I bought stroller connectors and hooked both umbrella strollers together. I could fit the luggage and both carseats and drag that behind while pushing the hooked together strollers. My DH says I put the "lug" in luggage so you can imagine what a pile it was. At least now, you can almost always check your luggage curbside-that is a lifesaver! I am sure whatever you decide to bring will work out with 2 adults and one kid-lol. Wait until it is just one adult and two or more kids-whew, then it gets hard. It was so worth it to me to travel though so I did it! Looking back, in some ways it was even easier than it is now. No matter what, you will have a great time at Disneyland!