I earned £7 a week in my first job and paid keep of £2 a week. When I got my next job at £9 a week, I paid £4 a week keep - and paid extra if I had a friend to stay.
With our son, because we were both working, we never took any money from him even though his girlfriend practically lived with us. When he was at college he worked at Pizza Hut at weekends but that was his spending money. Even when he got a full time job with a reasonable salary we never asked for keep. In retrospect, I realise that was quite negligent on our part because, instead of saving a reasonable amount of money each month, he just spent it.
I think we should have charged a small amount for his keep, and asked him to put a set amount of money into a savings account each month, on the basis that we would match it. That way, at a much younger age, he would have developed the habit of saving regularly and been much more responsible about spending money.
Having said that, we are quite fortunate that he has (surprisingly, given our lack of guidance), turned out to be sensible and reasonably good with his finances.
ninathenana £40 p.m. doesn't seem an awful lot for a young man's keep, but I don't think JSA is very much is it? So, unless you're finding it very difficult to manage on the money he pays you, I'm not sure that I would ask for more (though I do think he should be more considerate about the amount of snacks, etc., that he takes for himself). It's a shame he never goes out - it must be quite depressing to be indoors all the time. I know you said there are some issues that make it difficult for him to get work, but as I think someone else said, if he could get something part-time or even a few hours' voluntary work this may open doors to other opportunities. (I've just seen your last post and hope that he can get some help in finding something).