I wonder if it is possibly from chewing bars but I was more concerned he may have been stuck in a bell or something.
Is he eating ok? Other than the mark on the beak is there anything you notice which is different to normal for him?
My birds destroy perches like crazy too but I have two large birds and only one small bird- I am generally much happier to replace the little ones perches and I even buy him balsa rods to use as perches as balsa is one of his favourite things to chew.
Birds love chewing on wood to keep their beaks healthy so if you offer natural perches try to get them with the bark on as budgies seriously love to remove that.
I do not offer cuttlebone but I do offer a tonne of safe wooden things for my little guy to chew up as it keeps beaks and minds healthy and also keeps their bodies moving and happy.
If you are happy to share your cage set up we may be able to find how your little one has done this to his beak or maybe find you some solutions to stop him chewing the cage bars which is not the best for him.
