When you say he is on a balance diet, is this seeds and vitamins? Or is he on a nutritionally balanced diet like pellets? A loose seed mix does not provide the nutrition a bird needs. Also, Cockatiels do not drink a lot of water, so they generally will not get enough vitamins if the only vitamins they get are in the water. Loose seeds have hulls, so any added vitamins are lost when the bird hulls the seeds. He can have seeds in the form of our foraging diets. Our Nutri-Berries, Avi-Cakes and Pellet-Berries are nutritionally balanced the same as a pellet, but they are not ground up. Your bird can eat these, or pellets along with our foraging diets. But pellets and loose seeds will result in diluted nutrition. In addition, you should try to get him to eat greens, fruits and veggies. If he is eating pellets, generally you do not need to add extra vitamins, but during a heavy molt, this can be OK as long as the vitamins are not too concentrated.
This sounds like it might be a nutritional issue, but I would still recommend taking him to an Avian Vet for a check up. He should be making more progress as far as feathers growing back in. Here is a link to our feeding guide to give you more suggestions.
Bird Food Guide
Thank you for asking Lafeber,