Hi,
My partner and BIL both have been diagnosed with BPD but because they actually are very different, it has effected them job-wise in different ways. My husband is working part-time s/e and I mean part-time! might start at 12.30 and might do 2-3 hours, maybe 4 days a week (he has someone helping him also with work, which keeps him focused). On the other hand my BIL does not have a partner, lives with eldery parents and does not work - have to retire at 45 from his previous employment and the difference in the well being is amazing. So, for motivation etc and personal well being, to have something/anything to do a couple of days a week is most important for any person. I would suggest, as a first step to suggest to your BIL to ask his Doctor to put him in touch with a local branch of AWARE or a similar group and things may develop from there.
Also register with FAS, there may be suitable short-term course, to start with (maybe even a day course) as sometimes for a person with BPD to focus for a long periods of time, so there will be no pressure attached.