When there that small it is important too keep them fed. You can try different frozen foods but he's going too want live food. So guppies or small batch of Rosie's is what I would beef him up on. You can get him on other foods when he gets bigger. If he isn't eating dried foods or frozen then give him live. He will eat that. Sometimes they won't eat it in front of you. But if the feeders start disappearing then he's eating them.
easy. with Jars, above all. let it be hungry and start searching for food. throw in a couple of floating sticks.
If it picks them up and spits them it is a sign it recocgnizes it as food , but wants guppies instead. Another day without fod. Back to square one. You will have her eating in one week.
Problems always occur because we stress and want to see them eat. No healthy fish will let itself die with fod offered.