Is he not being forced to look for a job then if he is receiving jobseekers allowance?
You're right that it's probably entirely all his own doing, was he always so insular? Is there anything that he really wanted to do as a child/teenager etc that could be used to motivate him? He must be interested in something.
I agree with you, i think he has it way too easy if he is not paying anything towards his keep. He'll be sad but comfortable. Like a nest of misery. It's not doing him any favours.
In my opinion, if you have any money, I'd keep quiet about it but reveal later that you've had a gift of £xx and you'd like to use it for him to fulfil one of his lifelong wishes.
Whether it be visiting a different country, volunteering in africa with DFID, spending time at a commune or co-op creative set up. Anything at all really. He needs his world view broadened to jump start him into wanting more and meeting new people.
(I have experience in Further Education and met a lot of young adults who had 'life changing experiences that prompted them to return to education.)
Obviously there would be a cost outlay for yourselves but short term loss for a long term gain? And it is an obvious direct benefit for your son so I don't see how your DH could have an issue with it.