Well, I would say that you absolutely shouldn't have a dog at this time. Whatever pet he is kicking and throwing, for the safety of that animal, they shouldn't ever be alone together.
Many breeders of dogs screen families and will not adopt out a dog to families with kids your sons age and younger. Why? Because they play too rough, don't have good judgement, don't realize what they are doing at times. Your son sounds on the extreme end of this. But a lot of kids your son's age aren't great with young animals.
I would say that your son has not yet developed a full appreciation of nature--- it is a bird's nest but interesting to him. So he messed with it. Same with the toad. I would think this is not that unusual for his age. I'd read books about robins, toads, and other animals he may encounter in the backyard to help his appreication level.
good luck
I don't think it is totally unusual for a child to be reckless, but the concern is (as you say) that he is being reckless when another's life is at stake. What's going on in his life that would cause him to be so uncaring? Is he in a place where he is always being handled roughly by someone? (Sometimes dads are too aggressive with their children, thinking it's all in fun.) Another possibility is that he has some mild mental issue that makes him unempathetic. Do you have any feeling that this might be true?