Great that you're changing the light! Try some canned pumpkin without spices or baby food prunes. Try a couple more drops of oil.
Will he drink some water if you drip it on his nose with a dropper or syringe, or even your fingers. I'd give 2 baths per day. He might be dehydrated. Make the water a little warmer and a little deeper than usual, but do not leave him alone. The increased temp and exercise from swimming motions might encourage a pooper.
I would recommend switching the sand out for newspaper as soon as you possibly can. Loose substrates can cause impaction. later, if you keep him, you can switch the newspaper out for tile, or such.
Please let us know how he is doing.