Hi... If you go when
Disneyland first opens, you can start off with Fantasyland and try out the rides there first and then build up to the bigger rides for your son. If he's into pirates, "Pirates of the Carribean" will be a good ride, however, it is dark inside, so you might want to work your way up to that and discuss with him that it's okay... If he's into space "stuff", start him out with maybe "Buss Lightyear" and work your way up to the "Rockets", and then maybe "Star Tours" and "Space Mountain"...
There are many, many things that he will enjoy... not only the rides, but the shows, parades, fireworks (if you are there when they are having them), the colors, music, feeding the ducks and just being in Disneyland...
Also, if he like dinasaurs, take him on the train ride... it goes around the park and he can see dinasaurs in the dinorama between Tomarrowland and Main Street USA Station.
Oh, and one more suggestion that I think is pretty neat... if you are standing in a line, remember, he is standing around alot of adults and with him being shorter, it can get hot and uncomfortable as he can't see as well around people... you might want to hold him every now and again...
Have a great time, and don't let the crowds get to you... that's part of being at Disneyland... just take your time and relax and enjoy...
Have FUN!