What Kind Of Medication Do I Need For My Dog? He Has White Flat Worms In His Poop.

This is most likely tapeworms.  You can get an all wormer over-the-counter from your vet or pet store.  Ensure it is effective against tapeworm.  Worms in dogs can be potentially dangerous for humans, especially young children.  Adult dogs should be wormed every 3 months.  Ensure you practice good hygiene by washing your hands after touching the dog, remove any faeces from your yard, and prevent children from touching the dog of playing in the area where he defaecates for the time being.

You can go to the feed and seed store in your area and purchase NEMEX-2 it is  11.95 fere in alabama but it covers round worms and hook worms