Why do cockatiels lose feathers around their eyes?

I can't be completely sure because I haven't seen your bird. But, if your cockatiel is losing feathers on his body, he is probably plucking them out himself out of stress or depression. If he is losing feathers on his head, then another bird is probably preening him too aggressively. Either way, if it doesn't stop soon, I suggest you check with your vet on how to help him. :)