Boy that sure is bloated!
I would have suggested what you have already done - the peas and salts. Since the first dosing worked to begin with, I think a digestive problem is the cause. I wonder if he swallowed a small stone and it is blocking the digestive tract? Goldfish can do this, not sure about neons though :roll:
Judging from the pic I would wonder how old it is. It looks a fair size. As fish age they, like humans, can develop various health problems and perhaps that is what is wrong with the neon.
If it is otherwise feeding well and acting normally I would leave it. If it appears to be in distress, then euthanase it.
Keep up the treatments for a week, it might help if there is a blockage making its way through the system.