OK, interviewed my wife and (now 16-year-old) son and have some more feedback for you:
** Son feels Legoland is best suited for kids 7 to 11. Any younger and you can't take advantage of everything on offer. Any older and it's boring.
** There are basically four categories of things to do: rides, shows, hands-on Lego building opportunities, and tours of scale models of things built out of Legos. There is a recently added water park but it's not open in January.
The RIDES are perfectly tuned for kids 7 to 11 or 12, some (like the driving thing around Lego-Town) captivated my kid when he was 9 but will be out of reach of your son at 5.
The SHOWS are mostly fine for all, especially if your kid has Bionicle or Star Wars consciousness and would be amazed to see these characters live in the flesh, or doing stunts. There are some screen / theater-type shows that might be intense for younger children.
The HANDS-ON things are great for slightly older kids (Duplo pavilion possibly good for your son too) but some require day-of-visit signup as slots are limited. There is a Mindstorms robot-building thing that is terrifically popular, but maybe aimed at kids 8-10.
The SCALE MODELS of American cities and landmarks are wonderful but kids will tire of them before adults will.
Also, the place adds up to an intense whack of stimulus with so much noise, color, physical sensation, walking everywhere, Bionicles coming up to shake hands... By mid-afternoon Legoland resounds with the squalls of small children at the ends of their ropes and melting down. With tickets at $69 to $89 each plus parking fee and expensive food, you will perhaps not be happy if your 5-year-old son fades and wants out after just a few hours. When my son was 8, 9, 10 we'd line up before the gates opened, take the place for all it was worth, eat those apple fries by the barrelful, and get thrown out at the closing bell. He remembers Legoland with great affection but won't be going back until he's taking his own kids.