To lagitamate (SP) a child does there have to be a DNA test if the father is sure the child is his and both sides agree?

Nope.

Go here:

http://www.fultoncourt.org/family/packets.php#cont

Look for the packet entitled "Legitimation". Every form you need will be in that packet, including the instructions.

Those forms are for Fulton County, but you can change the county name on them, and they are accepted in all of GA's counties according to Legal Aid.