I can help you out with some info.
My little boy who is now five has been to DL three times and we're going again in four weeks. The first time we took him to DL and DCA he was two and a half years old, the next time he was three, and the last time he was four years old. He absolutley LOVES DL and DCA, but we did make a couple of mistakes taking him into a couple of attractions that frightened him. So here are my recommendations.
I would follow the advice of BlueBayou and take him on It's a Small World to see how he does in a darker ride. I also recommend taking him on several easier rides to break him in to
Disneyland and see how he does - (Jungle Cruise, Dumbo, Mad Tea Party, Autopia, King Arthur Carrousel, Casey Jr. Circus Train, Monorail, Disneyland Railroad). These rides are fun and easy on the nerves for him and you.
There are several rides and attractions he can go on, but they may be a little scary for him. You know your son the best and what he can or can't handle. If he doesn't get scared in the dark then he can ride most of the dark rides except for - "Snow White's Scary Adventure", which has some scary scenes that even seem scary to me. I also would not take him on "The Haunted Mansion". "The Pirates of the Caribbean" may seem a bit scary for a child along with the little downhill drop you experience on this ride, but if you point out the funny things in the scenes and go "Wheeee" with him down the little drop, he'll probably be O.K. with this ride (although it still will be far down his list of favorite rides at Disneyland because of his young age).
A Warning about any of the 3-D attractions - These will be too intense for your little boy! If you want to see these attractions, he can watch the movie without wearing the 3-D glasses, and make sure he sits in your lap because the seats and floors are interactive (they have movement) which give the movie a 4-D effect. We made the mistake of taking our little boy to "Honey I Shrunk the Audience" when he was four years old (we didn't know what we were getting ourselves into) and it took us at least six months to get him to go to the movies without being scared. "It's tough to be a Bug" at DCA is also very, very freaky, but "Muppet Vision 3-D", also at Disney's California Adventure, is a cute show and your little boy may actually like this one.
Another little warning - "Disney's Aladdin - A Musical Spectacular" at DCA is a great show, but towards the end there is a big snake in the show that may be a little scary for him. Don't miss the show on account of this - it's a terrific show - I just wanted you to be aware of the snake.
All the other rides at both DL and DCA that he is tall enough to ride should be fine, and most importantly FUN!
The weather this time of year is usually mild, low to mid 70's, but it can get cool in the evenings, very cool, so bring along a sweatshirt and/or jacket for everyone. Even if it's warm enough during the day for shorts and a t-shirt, by evening time it will cool down quickly and you'll want to put on pants and long sleeves. In other word, bring a little of everything.
I hope I helped you out a little and if you have any more questions, I'll be happy to answer them the best I can. Have a terrific time!
