Dont surprise him, because unless you know what he wants you may well be buying the wrong thing.
It is like surprising someone with a car and buying them a mini when they were expecting a rolls royce (or vice versa).
I assume he wants a whole new PC tower (which is the "case" and all the components in it).
As Ethel says, there are just so many different components that could go inside the case you may well be wasting your money.
Just to say again, DONT surprise him.