No... No distilled water!
Living organisms needs electrolytes in order to survive. Distilled water does not contain these electrolytes. Humans can't drink it (on a regular basis), and fish can't live in it....
Also, putting him in distilled water will make him retain
fluid. He'll end up becoming bloated, and could develop swim buoyancy issues because of it.
If you want to use a different water, get bottled DRINKING water from the store. Look for one that says something like "minerals added for flavor." These minerals are the electrolytes we all need to maintain good health.
But really, I think his regular (tap) water is fine. I don't think you need to change it. He's doing well in it.
Epsom salt has a laxative effect. If he swallowed a metal ball, he needs to work it out of his system. Epsom salt is the best thing for doing that. However, it needs a day or two to work on normal constipation. Eliminating a metal ball may take even longer than this. (I hope he can eliminate it, and that it doesn't cause a blockage.)
Why are you considering Vitamin C? I would hesitate to add a tablet since there are dyes, fillers and binders in the vitamin tablet. I don't know the effect of these on fish.