My ex and I are on a lease together, is he able to add people to the lease without my knowledge and consent?

Lots to unpack here, but let's deal with the simplest answers first: If your ex's mom strangled you (in urgent of your son or otherwise) she has committed a crime (many crimes actually - assault, strangulation, potentially child endangerment, etc.) and you should report the incident to the police immediately. Explain to to they police the whole context here - how she came to be in your home after getting a restraining order. Your top priority here should be the safety of yourself and your child.
Second, typically, stalking/restraining orders accommodate people so that they don't have to avoid their own homes (for instance when someone lives in the same apartment complex as the person against whom the order is granted), but in your case, I believe there would be a good argument for making her leave, yes. Stalking orders require repeated (twice is enough) threatening conduct, but you could still get a restraining order against this woman.
As from your lease question, I edited your query a bit to include landlord/tenant law. Whether someone is allowed to sublet, assign or add people to a lease can depend on the actual lease contract. But in general, landlords don't casually allow tenants to unilaterally add people to their leases at all. Moreover, you should consult with an attorney to determine whether his sending his violent mom to your home is itself a violation of your restraining order against him.
Do talk to the police and an attorney. Protect yourself and stay safe. Best of luck!

The above answer is absolutely right - first things first, do what you need to in order to be sure you and your son are safe. After that we sort out apartment and/or restraining order issues to help keep you safe. As to your ex's mother being added to the lease, I don't say it's impossible but it sounds highly suspicious and unlikely. In order to give a definitive answer, I would need to review your lease agreement and possibly talk with your landlord but normally residential leases do not allow subletting without your landlords explicit approval and your agreement. Are you sure she has actually been added to the lease or is your ex (or her) just telling you that? Regardless, if you get a restraining order against her, its enforcement should keep her out of your apartment. It might also give you grounds to break your lease without penalty if you want to go that route. The place to start, no matter what, is to meet with a local landlord-tenant attorney to review your lease and what your options are. Several of us are in the area and regularly deal with both landlord-tenant and/or domestic violence issues. Give one of us a call or send a email to make an appointment. We'd be happy to try to help.

In addition to what the other attorneys have already mentioned, I'd add one comment. If you have a FAPA (Family Abuse Prevention Act) Restraining Order against your ex, it probably also prevents him making contact through a third party. His mother moving in wouldn't necessarily be a violation. However, if he is using his mother to communicate with you, this may be a violation of the restraining order.
Also, I'd start with contacting your landlord to see if your ex's mother is actually on the lease. If she's not, she's trespassing and the police may be of some help. If she somehow is on the lease, you should make sure that your landlord sees a copy of the restraining order and knows that you don't want the other party's mother living with you.