Sorry to hear you lost your fish :-(
I dont think he died of depression. My guess is that he was already unhealthy to begin with, which would explain his "depressed" behaviour. All healthy young fighters should be greedy and active and it shouldnt take them 5 days to get used to new surroundings, should only take them 10 mins or so.
You noticed he had a damaged mouth at the beginning, maybe that had something to do with an illness or disease.
Also are you sure he was 2 months? that chart is quite inaccurate since most spawns grow at different rates. He could have been a runt, or the whole spawn could have been stunted