This "new" lawn you speak of, when was it seeded? What kind of grass did you use?

It was spray seeded about 2 months ago by the builder. I'm not exactly sure what grass it is but I could try to find out.

About the only thing you can do is mow regularly. It's a bit too young for weed killer, but mowing regularly will prevent most weeds from going to seed. You can use herbicides after the first few mowings, read the labels. Fertilizing in spring is generally done around Memorial Day, so now is OK. I would probably put down a half application of starter fertilizer, being sure to water it in. Most hydroseeded lawns are ready for their first mow in about a month when the grass reaches 3 inches. Why yours is growing so slowly could be any number of things. Since this spring was cooler than normal that is probably a part of the problem. If the landscaper used mainly KBG and fine fescues, those take longer to grow. If you haven't been watering often enough is another reason. Spring seedings are often fraught with problems like longer establishment times due to lower soil temps, weed pressure, and disease during the upcoming summer. You will probably lose some grass if this summer is anything like last years, but if it's cooler and wetter then you might make it through with minimal losses.