What a lovely friend you are and it sounds like your friend´s son will need some mentoring support. Please talk to The Parent Practice based in Clapham. One of the co-director enlisted a lot of support from her own son´s university for exactly the symptoms you describe. Elaine Halligan and her colleagues will be very well placed to help you. I have read Elaine´s fascinating book about unlocking her son's potential and can recommend The Parent Practice´s approach to supporting children of all ages.
Finally, I would also check that he has been assessed by an experienced Educational Pyschologist to rule out ADHD. A lot of people are unaware that ADHD people find motivation far more challenging than others. If any task is not very interesting to them, or they cannot see the point of it (that last aspect is often overlooked) then they just cannot summon up the energy or inclination to start/complete the task. This is because they lack the chemicals in their brains just to "do it anyway". In addition to this, they have a different sense of time so looming deadlines don´t necessarily spur them on either. I have direct experience of all this! Replacing the low levels of certain chemicals with medication as part of multi-strand strategic approach
can make an enormous difference, I was reluctant to medicate for many many years and now I regret it. The newly-diagnosed 16 year old son of a friend of mine said it was like being slightly short-sighted and wearing glasses for the first time.