get him eating live blackworms, and chopped up nightcrawlers and he'll explode in size, and health. atm my biggest M dayii is approx 21" and very outgoing and curiouse ( more like a fire eels in personality then a TT) also have a smaller one about 12-14" and my smallest is 6"ish... no clue how big they get but mine haven't slowed down much in growing. don't know if they'll get as big as armatus (3'+) but I'de say at least 24" isn't unreasonable.
bloodworms aren't very good food alone. as a treat its okay, but as a staple it's got little nutrition. mine are very open to new foods, much more so then most the spineys I've owned. krill, nightcrawlers, black worms, mysis shrimp, chunks of fish ( preferablly FW, white fleshed fish. ie walleye, bass, sunfish ect. avoid fatty fish like salmon, I also prefer wild caught over farm raised.. but thats getting very picky.), fw shrimp are all good things to offer.
btw- same would go for a TT or any spiney eel in general