What would you ask a 27 year old boomerang son to pay housekeeping?

We've agreed an amount that is just above a third but that includes part debt, I don't want to cripple him as I'd quite like him to move out ! But although we can manage without his contribution he's led a great free spirited life and it's about time he learnt how to manage a salary

Is he buying his own ‘health conscious , good food’ or do you pay for it? Does he provide his own washing powder? If not, then you need to be a bit more realistic.
I have a friend who rents a room to a single person, they pay £400 per month and provide their own food and cleaning products.
If he is strapped for cash , then a third of his wages plus you could get him to do some household chores for you? He could cut grass, go shopping for you, laundry, ironing.

He is an adult and has to start paying his way.

Can you not work out what he is actually costing you to 'keep', I think that is what I would do if it was one of my sons

How about a third of his earnings minus the debt repayment?

That sounds a lot if he’s only on a low salary and paying off debts I wouldn’t ask as much as that personally especially if he’s doing his own cooking and washing, but then only you know his business and abilities and both of your needs

A third of his earnings?