You have just described my son, except he was a hitter AND a biter. It will get better as he can communicate more but until then here are my hints:
We had some success with the following phrase:
"Mommy doesn't like to be hit. Hitting hurts. Mommy likes hugs." And then I would offer a hug. It took a lot of repetition but he reached the point where he would start to hit or bite and then just turn it into a hug. With the diaper change (as well as other things), I started offering him choices at about 20-21 months. "Time for a clean diaper. Would you like me to change it with you lying down or standing up?" (One note however, my son was the master of the third option...his response at 21 months was, "Sitting down, mommy." LOL...a little hard to change a diaper with a child in a sitting position)
There is also a book, "Hands are not for hitting" that might be good to check out of the library. Also teach him some sign language if you haven't already.
Addition: I just read Jim's response up above and I have to agree. My son turned out to be a sensory seeker. We have learned that the hitting and biting often times was a signal that he was over stimulated (other times it was just him needing to feel some control in a situation). Daycare was a horrid experience for him and we had to pull him out. When preschool didn't go any better, we sought help and received some help through occupational therapy. "The Out-of-Sync Child" is a great book to start with for figuring out if your child has sensory integration issues.