Yeah I'm still unsure too.
It has conflicting attributes.
Could be an immature plakat male (due to flaring but have short ventrals)
Or it could be a female, due to the largish stomach.
If I had to make a decision I would actually say male. Just down to the fact that it isn't showing stress or breeding stripes. A female in that container would probably show one or the other by now.
Plus it's anal fin (although a bit raggy), looks quite pointed. And it is consistently flaring at the other male.
Edit: Just thought...being the colouration it is...it could be female and be showing stripes but without you seeing them
Do you have any females? And a spare tank? If so could you pop it into another tank and chimney a female. See what happens. If it flares are her and builds a bubble nest then you can be pretty sure it's a male. Also if the female becomes eggy and shows breeding stripes you can be doubly sure it's a male.
Obviously you wouldn't want to actually release them and breed them. But it would serve as a test.