I've been several times, but a few years ago now. He's about the right age for it. It's a long day, and there's plenty to do there.
If you go in September, it may be when some schools have already started, so the queues may be smaller.
Take cagoules with you, just in case it rains, although it is in a dry part of the country, and take snacks and drinks.
The road up to Legoland from Windsor can get congested. Check other routes.
The view when you enter the park is spectacular. There's a right way round and a wrong way. There tends to be wasps.
I live locally, and when relatives went, they stayed in hotels in the area. Windsor is pretty close to other towns - Slough, Ascot, Bracknell, Maidenhead, Egham, Staines.
Windsor is lovely for a day out. It has a big castle (as seen on tv) , nice shops, river (stunning views if you go on the river), theatre, and parts of Eton were in the Harry Potter films.
It's only 20 miles from London.