Eclectus, CAG, BH Pionus, Maximilian’s Pionus, Quakers, Indian Ringnecks, Green Cheeked Conures, Black Capped Conures, Cockatiels, Lovebirds, Budgies, Canaries, Diamond Doves, Zebra Finches, Society F
I'm not expert but I think it caused by infection on his inner eye lid (they have two eye lids)
or maybe it caused by catching cold if he is sitting on the cage floor then you should keep him warm.