The feathers do look unusual, and this is not an area where birds can easily pluck themselves. The odd look of the feathers gives me the impression that he might have gotten something sticky on them and it wasn't comfortable, and he damaged the feathers by scratching at them or doing other things aimed at making it feel right. If you can handle him, you can feel the feathers to see if they feel stiff or sticky.
If there's gunk on the feathers, you can try to gently wash it off (especially if it seems to still be bothering him). If it's not bothering him, it's OK to just leave it alone and wait for him to molt out the damaged feathers.
I'm not an expert on feather diseases, but my impression is that this isn't a disease because the rest of his feathers look normal. But I could be wrong, so it wouldn't do any harm to see a vet to get their opinion. The biggest risk is that it might be psittacine beak and feather disease. PBFD can make the feathers look sort of like that, but it usually causes other abnormalities too.