If someone lives in an apartment and is not on the lease is he responsible for rent?

Legally speaking, he does NOT have to pay rent. The courts will see him as a visitor (legal term: transient). Even if he is added to an existing lease, both parties are responsible for paying the rent by the agreed upon due date to avoid eviction. No sense in having this freeloader drag you down. Just change the locks. Good luck! If he's living in your apartment, you certainly have the right to request that he pay or get out.