My son went through the same thing. When he started to lose his baby teeth we began regular dentist visits. They did a panoramic xray on his teeth. While his baby teeth were perfectly straight, the xray showed all these adult teeth that were forming and it looked like an absolute traffic jam. There was no room for them to come down. We did the expander starting in 3rd grade. At that age the palate has not hardened and the bones at the roof of the mouth have not knit. They can shape the bones as they grow. It's easier to expand now than it will be at any other time and it's a lot less painful. My son is 12 now and finishing up 2 yrs of braces. He will be finished with them before 6th grade is through. He'll be free from them for middle and high school and his teeth and bite will be perfect.
When I had braces in the 70's (back before expanders were invented) I had to have 8 adult teeth pulled so there was room in my mouth for the rest. I had braces from grade 5 through 11 and braces for 7 years is misery.
Orthodontists start younger than they use to, but they finish up much more quickly.