Our lease is until the end of the year, however, he must still take care of his financial obligations wheter he physically lives her or not.
You say you are in the lease until December?
It is possible that although he is legally required to pay, that he may not fulfill this.
This is a long time. Please consider your options here and what is possible and safest for you and your mom's physical and emotional needs.
You may want to give notice and mutually agree to release him from the lease or release both of you from the lease if he wants to leave.
This is the situation I am in right now. He decided to leave, however, I did not choose to release him from the lease early because I needed to continue to jointly share rent for financial commitments we had decided earlier in the year that affected my income and my decision on those matters was dependent on us continuing out the lease. He was not happy about this, felt trapped and potentially this could have been dangerous. He also tried to trick me into signing him out of the lease. While I didn't have indication that he could be violent previously, him feeling trapped brought out a "new" side of him and the situation became quite distressing. Fortunately for me, he has since moved out and is continuing paying his share of the rent, but I never doubted he would as he is serious about his financial obligations and his credit histories.