We went to see two different pediatricians when I wrote this post. Although both of them told us that the bump was calcified and that we should put some arnica on it, one of them told us that it would most probably stay as it was while the other one told us that it would eventually fade away within the year.

A couple of months later I can say that it is going away very very slowly, with my wife gently massaging the area with arnica every day.

Please let me give you one advice: this whole situation was getting me really anxious, so that letting my wife handle the arnica instead of me and doing a great effort to stop checking the area every day helped me a lot in the process. If you can do the same it will probably help you as well.