Just an idea, and feel free to ignore me

I tend to be a car seat geek. A lot of ladies on my "mommy board" site love the Evenflo Maestro for a travel seat. It harnesses up to 50lbs and 50in (and boosters far beyond that). It's FAA approved when used with the harness. $80 at Kohls online, could likely find it even cheaper. $67 for
Amazon if you have Prime.
Another option if he is over 40lbs would be a booster (either high back or backless). You could put that in the overhead bin, and just use it if needed for a taxi. You'd need it eventually anyway (carpool, transition from car seat to regular seat eventually, etc), and they can be less than $20. (As I said, I'm a complete car seat geek. My grandsons are still harnessed at 4yo and 5yo because I'm freaked out to move them to a booster, and thankfully their mom is equally concerned. However, I know we are in the minority. You said your kiddo's car seat is huge, so I'm assuming he's harnessed too. Not trying to talk you into early boostering. Just mentioning it as an option since "booster training" has to happen sometime barring some special need, and in this case, booster training him while you are sitting next to him may help, as you will be able to assure he remains properly positioned). Soapbox over
Of course, traveling with a full harnessed seat or a booster is not nearly as easy and/or convenient as traveling without a car seat, but safer if your flight is delayed...
Assuming you have 2 adults riding on DLRE, it will be $70 round trip SNA/Disneyland Resort area. A taxi would be about $90 (with no additional hotel stops, and no waiting on the next bus). A hired car is in the $150-200 range I believe, and could have the ability to have car seats ready (but not necessarily a correct car seat that will fit your child; Lansky has had an issue with this, and expired seats, within the last few years, but state they have bought all new seats).