Does a grandparent has the right of custody when the parents give up their rights?

My grandson lives in Springfield., Il. His dad lives in Springfield, Il. also. His dad (my son) has been trying to get custody of his son for a while now. My grandson's mother has given up his rights but my son does not want to but DFS is trying to force him. We love him and want him to forever be a part of this family. The family in Missouri see him when the foster parent allows use to...she's not very nice toward us. We take family pictures and have gatherings around his visit. My mother visits and used to take them on dates/fun places before Covid-19, and my son had visitation also. What can I do sowe don't lose him?