LA's nice time of year is usually from about April to March.
Just keep in mind, he'll want to go on "It's a Small World" 20 times, and then as he grows up will have no memory of the ride. You on the other hand will need theropy.
Free flight- sure do it. I went to Disney World at age 6 then again at 28. 10 would have been the right age IMHO.