I don't know why this is bothering you. And it sounds like he knows more than you're giving him credit for. He's using the term "Disneyworld", he knows about Epcot... He seems to have done his homework for his trips, and knows what he usually finds price-wise.
A good way of describing the total size of WDW property is that it's twice the size of Manhattan (or for me, a bit bigger than Tacoma, where I live). If those mean nothing to you, look up some closer cities to you that would mean something. Of course, a lot of property isn't the parks, it's just woods and roads.
He kept talking about all the "lands" at DL and comparing them to the "lands" at DW but it sounds like he thinks all four parks are one big park. This is not the first time we have had this discussion, btw. I HAVE tried to explain it to him before but I guess he just doesn't get it. ....
I know some people don't understand how huge it is, but when you show pictures and other stuff, you figure it would just click.
From a guide a trusted source on the boards wrote: " While physically smaller than WDWs Magic Kingdom, Disneyland actually offers 25-30% more attractions."
That might be what he's referring to.
I did a year's worth of study on WDW, I planned and prepared. And I was startled to find nearly proper highways on WDW property...I just did not expect it. (mainly b/c a city doesn't normally have highways running straight through it like that, or at least Tacoma doesn't) Some things have to be learned for yourself. And if he doesn't want to know it right now (or does know it and doesn't care) you're not going to push him happily into that knowledge.
LOL I have gone all over Florida...I can't wait to do a Florida only vacation again so I can go to
LEGOLAND!!! We didn't care for US/IOA, some of it was ok though. Haven't seen Harry Potter part yet maybe that will make it better for us. I have always wanted to go to Busch Gardens in Tampa.
And I can NEVER find anyone who wants to go to SEA WORLD with me....Even my 9 year old tells me it sounds lame....Ahh well, next Florida only i will force them all and they will see how fun it is(i hope)lol
You could take a southern CA trip, spend a nice chunk of time at DLR (if you wish to have your cousin use the proper terms for WDW, use Disneyland *Resort*, not just DL, because DLR refers to the whole place including DCA, while DL just refers to the park), travel an hour away to go to Legoland and stay in Carlsbad for the night, then drive another half hour south the next
morning to go to San Diego proper for SeaWorld...be sure to see the Pets Rule show, it's pretty awesome!
No need to go to Orlando for Seaworld and Legoland...
Oriented toward younger kids (12 and below)... so does that mean the LegoLand in CA is geared to older kids? If it is, I wonder how much will be cut out for the Florida one then...
My son is 7 now. We probably won't go back until he's 9 and our daughter would be 14. Right now he sees the videos for it and says he wants to go.
Legoland is not geared towards older kids...go ASAP. Unless your son remains a
Lego fanatic, then he'll enjoy it even in his 20s...but if you look at their website and the pictures of the rides...it's obviously for younger kids.
I was comparing WDW to DLR, so I was talking about how you can't fill up a week at DLR IMO so you have to venture out to other parks....
I entirely disagree with that. DLR is an incredible, awesome place if you really really REALLY give it a chance. We're experts at giving things a chance. Bought DVC before ever being at WDW (and just before The Grand Cal became available to buy as DVC), scheduled our very first family trip to Orlando to be 17 days long, upgraded to APs a few days into the trip b/c we figured we would need longer to really crack the surface, invested 4 days in Universal, on site, because we wanted to give it a solid chance. LOVED it all!
And when I got home, I cried because I missed Disneyland Resort so so so much; it's like home, and I miss it dreadfully (haven't been there since Oct '09 because the money is going to WDW visits for now). My family could *easily* spend *at least* a week just at Disneyland and DCA without ever venturing further away than the grocery store. It's that awesome.
It's entirely possible that the OP's cousin feels just like I do, but with the added problem of having a much bigger family. It's just the 3 of us for now, and it's easier financially to take a chance on a new place. It's also easier to get a majority vote in favor of venturing away from our happy place.

In a bigger family, not so easy...and much more expensive.
Since you are trying to teach him about WDW, you might like reading about DLR:
http://www.disboards.com/showthread.php?t=1162599 It's not totally up to date, so be sure to not have blind expectations based on some of the things he talks about, but it's still a good guide.
He has also written one for DL vets going to WDW which might be good for your cousin if you choose to pursue this (though I DO hope you'll educate yourself about DLR while you educate him about WDW!). If you send him the link, remember he'll have access to the Dis, and might just find your thread about him...
http://www.disboards.com/showthread.php?p=16541636