Yes, he can sue only you. Whenever more than more person may be liable, or resonsible for, a cost or payment or other obligation, the person to whom they are liable could choose to sue one, the other, or both--it is the plaintiff's (person suing's) choice. So a landlord could choose to sue just one of two (or more) tenants. However, if there were two tenants on the lease and only one is sued, that tenant could bring the other tenant into the lawsuit on a "cross claim" to require her to pay her share of the money (i.e. to indemnify or reimburse him for part of whatever he ends up having to pay to the landlord). The mechanism for doing this is for the tenant who was sued to file a cross claim in a complaint against the other tenant.