Give him a nice lukewarm pedialyte and honey soak. The honey helps them to poop.

Mix one part clear plain pedialyte to 10 parts dechlorinated or spring water. Add 2 or 3 drops of plain honey. Stir it up and put it into a nice soak bowl. Soak him for 15 minutes. It's like having a drink of hydrating vitamin water with a stool softener.

You can do this once a day for a few days. You can even put the bowl in their enclosure. Remove it after his soak is done. It should only be deep enough for him to sit nicely in. It's okay if the others get in also.