Hi op, I've suffered depression on and off for years, I'm 27. At your sons age, I did go through periods of unemployment and it is so so demoralising, I don't think there is anything worse. What got me through was a few good friends who I was lucky to have, my sisters and my mum did exercise some tough love. She would, gently, pester me about jobs, showing me the job ads in the paper, encouraging me to do applications. I found it highly annoying but I guess looking back it helped.
Depression, sometimes it's like a safety net, it protects you from engaging in real life. It's almost like you don't want to recover because then you'd have to start engaging with new social connections, new experiences which is scary! Especially if you've had knock backs in the past. If he likes reading, maybe you could encourage him to read something like 'The Road Less Travelled', 'Feel the Fear and Do It Anyway', even 'Who Moved My Cheese'!
Would he be interested in taking up an Open University course to help engage his mind into something positive? It would look good to prospective employers too. Is he one for doing sport? Could he join the gym? Like another poster said, her son was helped with focusing on driving lessons, can your DS drive and if not, could you sponser him with that?
There are natural remedies I find really helpful too, my main ones are Magnesium supplements. Google magnesium and depression, it's very enlightening. It also is supposed to help with insomnia. I use a liquid magnesium supplement and a mag oil spray as you can absorb it through skin. I dont think I could do without it now. It's a natural laxative too as you said he suffers in that area. And a vitamin D3 spray, particularly if hes not getting outside.