There could be a combination of reasons why your horse is resistant when you try to pick up his rear feet.
One important factor that would be helpful to know is: Could you pick up this horse's hindquarters before you had him castrated?
The following "Ground Handling Video" in YouTube provided by eXtension describes how to safely pick up a horse's feet:
http://www.youtube.com/watch?v=cT4ASkfRz4I&feature=channel_page.
Some important factors to consider would be:
1. Have someone else with you when you work with this horse.
2. Have someone hold your horse rather than just tying him when you are trying to pick up his feet.
3. Make sure the handler is practicing proper safety by standing on the same side of the horse as the foot you are trying to pick up.
4. Take small stages of progress as valuable, and reward the horse for letting you touch him down the leg to the hock and then to the cannon and pastern.
5. If the horse lets you pick the foot up for only a moment, be sure to reward him and do not press the horse to hold the foot up longer. Let this be a gradual progression since the horse sounds as if he is waiting for you by kicking.
6. Try doing many routine activities with your horse to enable him to gain your trust and give him a break from trying to pick up his feet. Then gradually try to reintroduce the task when other activities and tasks are mastered.
7. If the horse strikes/kicks with the intention to hurt you and moves toward you while kicking, you may need to consult a professional trainer to observe the situation and get the customized advice that we cannot provide over the Internet.
