How could I get guardianship and a name change for my grandson?

You can seek guardianship of a minor by filing a petition with the appropriate state court. (In some places it's the probate court, in others family court). It's a good idea to find an experienced family law attorney to advise and represent you. If either or both of the child's parent have custody, it will likely be a difficult fight.

You have to file a third party custody action if the child is living with you. If he has been living with someone other than you, the chances are slim the court would give you third party custody when he's already in another home that is stable. The court is highly unlikely to give you permission to change the child's name as there is no good reason to change the name of the child from father's to mother's when he isn't living with either one. Get a lawyer to deal with the 3rd party custody situation.

Your question raises a number of issues, have you been appointed guardian or conservator, have you adopted this child, if not you're going to have to have the parents' permission and most probably cooperation in order to change the name. I would suggest that you contact an attorney with the details and for advice as to how to proceed.