I think I saw if he sits out entirely he can enter next years draft.
Like someone else said, I'm betting that it doesn't work like that and Bengals will still retain rights to his contract. If I remember, that how it works for baseball players that get drafted to the NFL but don't immediately play there or they don't play at all. Pretty sure if a team drafts said player they retain the rights if the player does decide to join the NFL at a later date.
[EDIT] Never mind, I must be thinking of players in the past when guys like Rocket went to CFL and later rejoined NFL. I just found this on ESPN's website under NFL Draft rules:
Can a player refuse to be drafted by a team?
No. Players cannot prevent a team from drafting them, but they can refuse to sign a contract upon being drafted. If that happens and the player's rights are not traded, that player would be back in the draft pool the following year.
I guess the question is if he's truly out of his contract or not because of off the field issues?