I don't know about a GAC at
Legoland, but I just went there last week with my son who has high functioning autism. He is almost 3. I honestly don't see a need for a GAC at Legoland. The park is very small and quaint, and the lines are not anything like what you will find at a Disney park.
How old is your nephew? Legoland has a lot of rides with minimum height requirements, so depending on his age/height, he may or may not get to go on most of the rides. A lot of the rides that have longer lines are minimum 42-48" to ride.
The "anyone can ride" rides rarely have lines over 15-20 minutes. Some of the rides have a play area with legos where the kids can play while the adults wait in line. This is a GREAT feature! The park map shows which rides have this feature. The kids are in an enclosed area with
lego tables and they can just play within very close reach of the rest of the line. The lines sort of snake around in a small area, so you can see your kid the whole time. In addition, one parent can play with the kid while the other waits in the line (esp. useful for autistic kids who may not "stay put").
Most of Legoland is interactive. There is a LOT to do that does not require waiting in a line. There are many open play areas and places to explore. In addition, there are lots of little "lego/duplo" play stops located throughout the park, so your nephew can have some time in a relatively quiet indoor area to unwind.
Keep in mind that lots of locals visit Legoland on the weekdays. They bring their toddlers and come in the mornings, but usually are gone by 1pm or so. This means that, upon park opening, there will be HUGE lines in all the rides in the front area of the park (the storybook boat, the safari jeeps, the coast cruise, the dinosaur coaster, etc.). If you return to these attractions around 1pm, they will mostly be walk ons. I would say skip over any attraction that has a line longer than 15 min and come back later in the day.
My son LOVED looking at the Miniland models. Some of them are interactive as well, pushing buttons to make stuff happen. This is a free roam area, no lines at all, you just walk through and explore. He also spent a lot of time in the Explore Village. There are fountains galore here (interactive as well) and a huge playground with slides, houses, tunnels, and a train ride. No lines here either, except for the train, which was a very short line.
I don't think you'll find you need much assistance, as this park is not nearly as overwhelming (sensory wise) as
Disneyland. I think you'll be pleasantly surprised.