First, check your lease. Your lease controls. If your lease states that you can have one occupant at your discretion without landlord approval, then he can stay without landlord approval. If the lease states that you can have one occupant but with the landlord's approval, then the landlord's approval is needed.
If the lease is silent on the matter, common law doctrine controls, which states that the landlord has the right to approve or disapprove. If so, then it is up to him to decide whether or not to approve him due to his criminal history.
Now, check your lease more. If your lease actually does not disallow subletting, then there may be a way around the restrictions above. Let me know if you need more information on this.
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