Oh dear. I had a constipated oscar once. Didn't turn out well
Here's what I would do:
Turn off lights.
Add melafix. When the fish breathes in, the melafix will enter his system and ease his insides
And, of course, water changes. I've also heard epsom salt also eases it, but never tried it myself.
LIke i mentioned, i've messed with this problem, and I didn't win. What i think happened it the fish swallowed too much air while feeding and it built up, then clogged his intestines/stomach. I tried everything, peas (which she refused), melafix, no lights, etc. Eventually I euthanized her with clove oil, she appeared to be in a lot of pain.
Hopefully you'll be more lucky.
Art