thats a tough situation, do you have baby cereal? you can mix that with warm water as well (temporary formula), and let him eat the seeds! it's ok, you can change that later, food is what is important right now. Some babies regress...that is always the chance you take. but it's ok.
load his cage up with food! what every you have, use a lot of it. apple sauce with cooked rice, mash, cherrioes, sprouts.....whatever you have. Baby food, oatmeal......
give him nothing else to do but play with food, hang it, put it in dishes.....go crazy. use enough for 4 birds, it will be wasted but that is ok, it will make him explore it, and hopefully eat something.
If you have some sort of hut that can go on the bottom of the cage that will help, when I raised caiques I found that they were very comforted by having the ability to retreat into a hut. They ate better and ultimately became more confidant birds. you might want to try that. You can use a small cardboard box, or a plastic guinea pig hut, both work well.
Cover the cage half way, fussy babies often lose the focus and forget to eat.
leave him be when you give him his meals, you will only distract him from eating, and provoke his begging behavior.
I don't know what to tell you about going away, but the above needs to be done
