What is the usual procedure/outcome for a child whose baby teeth are very close together?

My five year old still has all of his baby teeth, and they are packed very tightly into his mouth. Usually when a kid is his age, the baby teeth have little gaps in them to make room for the adult teeth that will soon be coming in. His are very straight, but so close together that even flossing is sometimes a challenge.

The dentist said that it’s something we’ll have to address as problems come up when his adult teeth appear. We didn’t discuss it in more detail, but I am wondering if anyone else has had this kind of dental problem, and how it was resolved. Thanks!