Q.When can a person spray weeds in new grass
A month ago, I reseeded an area of my son’s lawn in a fescue mix grass seed. The grass has came up very good, about several inches tall. But the people who lived in the house before tilled but didn’t do anything. Now there are a ton of weeds, more weeds than grass. I am afraid the grass might get chocked out. Can a person spray these weeds without hurting the new grass?

You can use a post-emergent herbicide. Read the label carefully to make sure the product targets only broad leafed weeds, but not grass. This article will help you: https://www.gardeningknowhow.com/plant-problems/weeds/post-emergent-herbicides.htm
You can also seek advice on the best herbicide to use from your local Extension Service. This link will help you find one: https://www.gardeningknowhow.com/extension-search/