I don't think you are being unreasonable in your expectations. However think of it from his perspective, if he does not work, you will still pay for everything, so what is his incentive to work?
I'd recommend that you tell him he needs to work this summer, so that he will have spending money for the school year next year (assuming that is something you pay for, or for books or whatever it is you want him to cover the cost of). Let him know that you will not be paying for those things if he does not work. That if he works over the summer, and ends up coming up a little short, that you'll help him out, but that he is an adult and needs to take the incentive to start covering his own bills.