Looked it up. Something similar; person's cichlid was eating fine, acting fine, then bloated (no dropsy, still eating and active) another fish was affected in the same tank.
Someone's post to them:
"Hi Rusty! WOW, that's really gross, but it's the closest thing I've seen to lympho for a while. The lympho case my fish had (if it was lympho...) was mainly around the fins and fin bases, only in a small number did it affect the body or head, I suspect though that any body tissue is a target. My fish survived but it took many months before the lumps went as fast as they came, if the fish in the pic can't eat properly then it will die." (Poster me tioned this as the fish had it not just on fins, but also the mouth)
On a koi forum another one shows pics closer to what your boy has (I looked up "fish fins lumps" in google) reply was:
"I put the fish in a sick tank...I took a scrape of one of the spots, didnt see anything..Not sure if it might be just trauma, I tried anti-bacterial agents for a week...After that didnt work, I tried anti parasite medication that I happened to have, and that didnt really work...I ahve the koi at 75 F and 0.2 to 0.3% salt, still the spots are there, seem to be gettgin a little better perhaps, it has been about 3 weeks.
I think it might be viral. The koi is acting just fine, very active in fact. I have looked and it might be "lymphocystis" and a possible treatment might be acriflavine, but, according to one source it might work..."
Then it also mentions parasites. Someone mentioned flukes, another said his seller of his koi said he commonly saw it. I'm not posting the procedures as koi are huge and bettas are small :) so perhaps this narrows it down to a few possibilities. I've never dealt with flukes or other external parasites - OFL has mentioned it in her post however and how it can be removed.