The lease needs to be read.  I am about 99% sure, though, that it probably says that you are jointly and severally liable for the money under the lease, meaning that you will be responsible for the entire amount - yours and his.  Yes, you can resolve it by paying it all - or negotiating with the debt collector - and then suing him for his half of the rent owed.  I would indeed speak with someone on the matter and bring to his the lease to be read.  I know that it is hard to deal with debt collectors.  You feel so vulnerable and out of control of the situation.  So maybe go and see if a tenant's rights organization or a free credit counseling place can help to make the initial call.  It will not go away.  Good luck to you.