Yes on the water bottles and snacks...I don't even remember there being a bag search there.
FYI there is a big water play area inside LL and several rides that can get you soaked, so towels and a change of clothes (for DH and DS) would be in order. There is no way to watch your young child closely in the water area without getting soaked yourself (think huge bucket of water that dumps over the entire thing periodically) and it would be a pretty miserable ride back on the bus drenched. We usually avoid that area all together, but it might not be so easy to convince your DS to avoid it and the soaking rides too.
Other than that, lots of sunscreen, and check out the website ahead of time to know what your DS most wants to do. Some of the workshops you need to sign up for first thing or they fill up, and all the rides are slow loaders, so the lines get long and move slow...make your priority list and head toward the back of the park first and work toward the front so you work opposite the crowd.