C_G
Having been in a similar situation as your Son not that long ago, I have to say take a look at it from his side of things too. I can't thank my stepmother enough for being patient with me (and bearing in mind I'm not even related to her) for putting up with me whilst I was unemployed. We had previously not had the best of relationships, but after my Dad left her and then I lost my job, we began to get on a whole lot better. OK at 28 (at the time) I was a lot more mature than your average youth that has just moved back home, so was very conscientious about making sure I left things clean and tidy, and trying to do my bit around the house, even if that was primarily just not making any mess in the first place.
One thing I will say that I think made the whole thing work was that we did give each other plenty of space (quite hard when if you're unemployed, even leaving the house is an expense you can ill afford!), but also that I was VERY actively seeking work. When looked at from her side, I could imagine that if I was being rather slack about finding work that it would be absolutely infuriating, but that I was being very proactive about it made her a lot more understanding. To be fair though, I'd been totally self sufficient for a good few years at this point, being at home and unemployed was soul destroying for me, so I wanted to sort it out ASAP!
A Few ground rules and some trust are required... And not the sort of "You must be home by 11" kind of ground rules, but just about house cleanliness and standards, sharing the workload, and obvious evidence of having looked for work actively.
Good luck to the both of you, I hope it all works out!
Oh, and to be fair, I'm sure you're a cool Mum anyway, riding bikes and having loads of mates over for parties etc... Way better than most!