Bettas don't bloat because of stress, unless the stress causes a weakening of the immune system enough to cause an infection.. either way it's not a direct correlation.
To me it looks like more than just a belly bloat.. is it fair to say his BODY looks bloated? I think at this point I would not be treating this naturally. I would use epsoms along with a good internal antibiotic like Kanaplex, but up to you.
I've never fed random veggies to bettas before but I have fed half a shelled pea and never had a problem. You want to use fresh or frozen, never canned. You also want to use organic if you can. Nuke the pea in a very small amount of tank water for 10 seconds, shell it, half it (it will come into two pieces once you get the shell off) then break the half into tiny little pieces to feed. It is very effective if it's only constipation but that doesn't look like constipation to me..