2 minus 1 leaves only half the rent

[Begonia]

My son splits rent in an apt. with another. Month to month. The other leaves 3 days into the month and doesn't pay his half. My son is stuck and tells the landlord that he is looking for another roommate. Landlord says his half is good for 15 days only then and he is out in half the month (this month!). In addition, the security deposit is paid by the incoming person(s) to the leaving person(s) as new people move in and out over time. Is this right? What are my son's options now? Can he be forced out in just 9 days? Help, please.

 

[jacksgal]

HIs options are pay the full rent and sue the former roomate for his half