So it's definitely possible to find "Nearby" instagram accounts, and if someone knows where you live, it is very possible that people can use this feature to find the account. I would recommend the following in order to protect your instagram account:
- Reset the passwords on any E-mail accounts you may have, ensuring that the new password is strong and randomly generated. If it gives you the option, force re-authentication for all access (it might call this force logoff all sessions, or clear sessions)
- Reset your password on Discord and any other social apps you use, again with randomly generated passwords. Same goes here for forcing all sessions to logoff.
- Once this is done, you can try to work out how he is finding newly created instagram accounts by, for example, creating a new account on your phone with a randomly created name and information, then creating one using another device (a public computer, a friend's computer, etc) and see if he finds that
This definitely sounds like generic internet stalker/creep behavior rather than any actual hacking
but always better to be safe than sorry!