By the way you describe it you ARE employing him and the Revenue would be within their rights to insist on him being treated as one. If he works for you, only for you and you decide when he works then he's an employee.
Any particular reason why you want him to be separately self employed? The obvious solution would be to make him a partner in your business. Then you can contrive the profit share however you liked at year end to be most tax advantageous.