Will I be sinful for what my father sees after I created an Instagram account for him on which he spends long hours, and what should I do about it?

There is no sin upon you for what your father views, if you did not know his intention to use the account for forbidden things when it was created. It is forbidden to think ill of any Muslim, especially one's father. There is no objection to reminding him and advising him to benefit from the account in what is useful, and warning him against what is forbidden, while observing proper etiquette and showing humility towards him. You can also remind him indirectly by sending him admonitory clips.