Well all I will say is obviously keep the whip away from the back and kidney area's as that can seriously hurt someone - In an internal damage sort of way...
Apart from that, just build up slowly so that he has the experience of you starting softly and then harder and harder....And obviously make sure that he has a safe word so that if it gets too hard for him then you can stop immediately...
Hope you have fun....I always envy people that can do Dom...I'm naturally submissive so I'm rubbish at Domming...LoL....
Do come back and let us know how it all went...