I second Falconez's suggestion. Make sure the water is no deeper than up to his chin. Make it a little less than that and disolve 3 to four drops of honey in the warm bath. Have a cup of warm de-chlorinated water ready as well. While he is soaking to help stimulate a bowel movement pour some warm water over his back and then make the stream your pouring hit directly on his vent and flow over it. This should help. If he does not go than just continue the soaks, but the rest should not have honey in them. You only do the honey soak once. Repeat soaks daily until he goes.

If he starts to become stressed from being handled and soaked skip a day. Keep a close eye on him.

Keep us posted.