Hello!! Your nephew will love LegoLand!! I have a few pointers that may help you make the most of your day!!
The first, is arrive at opening...give yourself 15 minutes to buy your tickets...and then get in line at the turnstiles. When you get thru the entyway and can turn right or left, turn right. If you turn left, you will go into the area more geared for the smaller children. All good to see, but better to hit it when the toddlers have pooped out and the crowds are lighter in that section.
Since he isn't a big coaster fan, no need to head right on over to the technic coaster (my DD's favorite since she was 4!), but maybe the Coast Cruise...a view of miniland, then go through the imagination zone, make a left right past the restaurant and view miniland or better yet, cut through miniland to reach Pirate Shores. The rides are completely not scarey, Splash Battle is a hoot, but a slow line, so want to do that one early!!
Remember a suit for at least your nephew...the water park is super fun on a hot day!
Maybe do the rides, SB, a log ride and Cranky's Challenge and head off elsewhere before he gets soaked. From here you can go to Funtown (Helicoptor ride/skipper school/Fun time Fire Academy (this one is about 3 years old so you may not have done it...you race another team by pumping your firetrauck up to a fire, dousing the fire and rushing back to the start) and the Driving school which is great fun!! The Kid Power Tower is a blast too. A ride with long lines that is actually pretty much zero fun is the Sky Cruiser...it looks exciting...it is not. Everyone I've ridden it with is diappointed.
Try to catch the 3D movies in the Imagination Zone, the ventriliquist act in Fun town and the Fireman show in Funtown. They are all entertaining.
Chill out times can be found in a walk through exhibit in Funtown called the Adventurer's club, may spots with Legos toplay with (great one in Knight's kingdom) Games and build your own
lego racer in the Imagination Zone.
Oh, fantastic play structure in Knight's Kingdom!! Another must do!!
Okay, all disjointed...probably not even all that helpful...wish I wasn't out of maps to send....have fun!! If this diatribe brings up more questions, feel free to ask!!