Clearly, he wants to chew on something in that spot. You can have someone weld the bar back on. However, being a cockatoo, he might fixate on that spot and bare metal can be problematic if they start chewing. You also have to be careful that it's a decent welder that knows what they are doing and doesn't leave little rough bits of metal that can be chipped off and ingested. Personally, I'd have a long term goal of getting the bird into a cage with thicker bars. My large cockatoo is in a cage with .25 inch thick bars. The cage you have looks like it might be more suitable for a conure up to maybe a goffin's sized bird, but not a medium to large sized cockatoo. Short term, I'd go to the lumber yard and get a 2x6 untreated pine board cut and measured to go vertically from the cage grate to the top of the cage when fitted inside the cage. Drill two holes in it and bolt it on with some bolts, washers and wingnuts. If he wants to chew something in that spot, this gives him something to chew. Also, most cockatoos can't get through a 2x6 all that fast. If he's hard core chewing, you might have to replace it every few days or weekly. I'm guessing most cockatoo will take a while to get through it. Or might go through phases where they chew through a couple of them fast and then start to slow down when it stops being new or hormones settle down.