A crop/whip is simply an extension of your leg aid. Of course, any tool in the wrong hands can be used cruelly.
If you are working on the flat to get him more responsive to your leg, I would recommend a dressage whip instead of a crop. This will make it easier for you to use the whip to tap behind the lower leg, which is the aid you are trying to reinforce. I typically only use crops or bats when jumping, when you need more control over the horse's shoulders to prevent him from ducking out of a jump.
Of course, you will want to acclimate your horse to the dressage whip (or any crop/whip for that matter) if he has never had one used on him before. Otherwise, he may kick out in protest if you spring it on him without warning. If you need to get him acclimated to a whip, start by touching it all over his body, starting with the lower legs.
A word of caution: the dressage whip does not have a wide leather piece on the end like most crops do. The skinnier the whip, the more painful it can be if used inproperly. A tap is all you should need to reinforce your aids.