Hi A.,
You didn't mention if you were giving him water. Make sure he gets plenty. When a baby is breastfeeding it is not as critical but as soon as food starts, water is essential. Constipation and hard stools can cause irritation and also toxification within the colon that can cause problems down the road.
If you are giving him enough water, straight prune juice is the route to go. It will stain his clothes but most babies love it and it has enough fiber and is healthy for him. Avoid corn syrup as it's as bad as honey. Corn Syrup and Honey can contain botulism spores that his digestive tract can't handle yet and can be fatal. After the age of 2, they should both be safe.
Don't worry. Keep asking questions. Trust your instincts and you'll be fine.
God bless,
M.