You can buy all the Lego, in all the formats - obviously they want you to buy the branded, licensed "sets" but if your lad wants to blow his wad on yellow 2x8 bricks only, that can be done.
Dunno how much fun a 2.5 year old will have, they are a fickle bunch in my experience, but it is generally aimed at younger kids, so there will be plenty of stuff you can go on. That said, there will probably be a few things that a 6 year old could go on that a 2.5 year old can't.
Usual theme park format - the food and refreshments you can buy onsite are over-priced and not great quality, you can either live with it or take sandwiches (I'd take sandwiches).
My two actually really enjoyed the lego town sets (not expected, they both went mainly for the rides). There was also a fantastic Star Wars area which we all liked, but I'm not sure if that was permanent or not, so it might not be there?
A few of the rides will involve getting wet, there's enough that don't that it won;t ruin your day if you want to avoid them.
Just to check - you are talking about the proper Legoland in Windsor, aren't you? All the above applies to that. All I've ever heard about the place in Manchester (Trafford Centre?) was that it was massively disappointing...