Welivefordisney: Let me try to remember (I've only been to Disneyland once, but...) first, as others have pointed out, as long as your son can hold on with his hands where necessary, he should be fine. Note that Mulholland Madness at California Adventure is very similar to Primeval Whirl. Space Mountain is great - instead of three-person vehicles those each seat between eight and twelve people (it doesn't vary, I just forget). But what makes it great is that they can move a ride vehicle off-line, aka off the track - so there's no time restriction on boarding. Also, while it turns out I didn't need it, I was told the same could be done on the Matterhorn.
Note that while, just as at Walt Disney World, as many ride queues as possible are wheelchair accessible, the ones in FantasyLand generally aren't. Just look for the wheelchair symbol with an arrow sending you in the right direction, or ask any CM.
amycishere: It's not actually a matter of being strict or not - it depends whether your chair will fit in the space allotted. I know at Ellen's Energy Adventure, the wheelchair Guest can roll up a ramp right into the ride vehicle. A wheelchair will fit; an
ECV won't; a power chair may or may not, depending on its size.
eta - couple of attractions I didn't notice mentioned.
ºOº Splash Mountain does have stairs to get to the loading area BUT also has a wheelchair 'diversion'. Follow the path with everyone else until you come to a gate with the wheelchair symbol and a CM.
ºOº Swiss Family Robinson Treehouse is entirely NOT wheelchair accessible. There are many, many stairs.
ºOº Jungle Cruise requires two big steps down, onto the seat then the floor, and the reverse. However, there IS one boat that it wheelchair accessible - the Guest remains in the WC/ECV and boards via a temporary ramp.