How much baby prune/apple juice for constipation?

My son has occasional constipation now that we have him on formula. Can anyone tell me how much prune/apple juice they have given their children to help them go? 1oz? Diluted with any water? I bought the Heinz baby brand. I am more comfortable giving my son juice rather than sugar water.

I depends upon how old he is, and how is body reacts to it. If he has never had juice before, you would want to water it down, 1/2 juice, 1/2 H2O. You need to experiment, to see how much he needs to produce a bowel movement. I would start at 2 oz. (1 H2O, 1 juice), and work up to 4 oz. (2 + 2).
If that doesn't work, you should talk to your doctor about other options, such as changing formula.