Took 4 year old and 2.5 year old today - it was quite busy given still in term time, guess a lot of schools must have had inset days.
Which rides your DS can do will depend on his height. 4 year old whom I nanny has recently got to the 1m mark, so can now go on The Dragon rollercoaster and loves it
Agree with the others that heading to the back of the park and working forwards is the best way to do Legoland. Some areas of the park have later opening times to others, not sure if that is the case in peak season or just off peak, but worth keeping in mind (Adventure Land and Knights Kingdom have latter opening times).
Take the hill train down if it isn't too busy. When it is busy (it was today), we walk down, through miniland, keeping as close to the route of the hill train as possible.
Today we went to The Dragon, had a coffee and muffin (children had a snack and drink from the picnic basket) at Crossed Ribs BBQ, then went on Carousel, Chairoplane and Orient Expedition. Then had picnic at the City Harbour show area (didn't get another coffee, but we could have done from Ice Cream & Doughnuts), then went on to Dino Dipper... Dino Safari is popular but today had a large queue so skipped that one out. Then up to Duploland for a play in the playground and trip on Duplo Train. Then it was back home as getting hot and irritable.
When we went last week it was quieter and we managed 10 rides in the day, not bad at all for us.
Tend to find that lack of things to do isn't a problem, it's more that we run out of time. So make a full day of it.