OP one of the psychiatrist who devised the diagnosis Borderline Personality Disorder decades ago now believes that patients who were diagnosed with BDP in fact were suffering from trauma. Trauma is able to be treated by a number of therapies, without drugs. Whereas borderline patients faced a lifetime on drugs. this doesn't like borderline to me.
There are a lot of things you can do to help your ds with how he is feeling and these are some ideas - these are just ideas though:
Going out for a long walk, say 2 hours or so, at weekends and talking to him (and other family members) the whole way will be great for him and for you - the exercise will make you all feel better, and although it doesn't sound appealing once you are on it and enjoying each other's company it will hopefully be great and it will give you a chance to hear him chat naturally which might give clues as to why he is feeling as he does.
Talk about his feelings a lot
If he likes art, get him to do a lot of it at home - good for selfexpression
Encourage other hobbies - sport, music
Go for jogs with him - the feel good hormones will raise his mood if you go regularly
Make sure he has support he needs to do well at school as that will help with selfesteem
Pre arrange things to do together for him to look forward to such as films you will watch together
If he doesn't want to be seen by people maybe take him out somewhere a bit further away,with the family, somewhere he might like to see and where he is unlikely to bump into people he knows
The aggression is an indication of how he is feeling but still important that you talk about what is and isn't acceptable behaviour.
Hiding as you describe is a temporary solution but will likely cause more problems than it helps, in the end - giving your dc more tlc is probably better it will pay off in the long run - though I agree it can be very tiring.
13 is still v young for some boys I am sure he will be fine. Maybe imagine the kind of life you'd like him to have as an adult, talk to him aqbout it, how he feels about the future, what he'd like to do as a job, and work out what skills you could help him with now to help build towards it