Hello ATXSmiths,
It is difficult to know what to do, how much to do, and if to do.
Remember that safety is the number 1 concern. I encourage you to let go of any guilt you might feel for looking through his phone. He is only 13 and needs limits and boundaries, close supervision, and unconditional love and support from his parents/family.
If your stepson13 was not suspected of having BPD would you allow him on those websites? If yes... .it is best to monitor rather than forbid as forbidding forces behaviors underground. Having a cellphone, online access, etc... .are all privileges that need to be earned through responsible behaviors. If these privileges are being abused then he can earn them back through behaving responsibly.
Talking about self injury is important... .a conversation based on compassion and understanding rather than fear and confusion is paramount... .no judgment. An honest discussion about intense emotions and how to use words with an understanding and accepting person (like you

) lets him know you are concerned and there are healthier ways to cope.
At 13 his privacy is important and you need to know he is keeping himself safe. Setting a limit in advance of crisis... .like... .if we are concerned that you may harm yourself we will need to be sure you are ok and will want your door left open until you are feeling better able to cope.
Isolating is one way that our kids feed their illness... .when my d14 came home from 10 months of RTC isolating was not allowed. Interacting with the family was a requirement for her continued recovery. Having meals downstairs every day, conversing about the mundane daily after school, doing chores
together no headphones in the car (to encourage conversation) were some of the things in our home contract.
Without skills your stepson will struggle with all of these... .the most important thing you can do is to
Validate him set
boundaries and limits gently and firmly and accept him unconditionally. It would be wise to have the same limits and boundaries (ages taken into consideration) for all the children as the new school year approaches. This will be both supportive of your stepson while increasing the overall well being of the entire family.
lbj