S.H.
Just do what your son likes.
If he could care less about an outing or sleepover, then I would not.
But, maybe cook his favorite dinner or dessert????
That is also doing something nice for him without breaking the bank.
You could get him 1 yu-gi-oh card, or DS game.
Or get him a gift certificate for the store. And he can pick one himself.
Just ask him, what he wants to 'do'.... beyond a birthday gift. But if he's not into anything, then fine.
But that you'll also get him something special for his b-day gift.