How should I handle a dementia patients physical abuse?

Try relaxing first.Clients pick up quickly on your mood.
Don't go in with the sole purpose of "changing the diaper."
Sit a moment, make conversation about anything.Do a little dusting, whatever.
Remember that despite their dementia they still do suffer the embarrassment and indignity of not being able to attend to these very private moments of toileting.Many can not admit to themselves they are incontinent.
Then try something like" Let me help you change" Get the diaper ready and if he is able let him help.Take your time.Don't rush.It's a struggle for them.Talk about the weather etc.It helps distracts them.
Be sure that when you put the diaper on it's comfortable.He may be experiencing pain, pinching, chafing or other that has made the change a dreaded experience.Many patients experience "pain" or anticipation of pain and punch, bite or kick out before you even touch them. Touch is important.Touching their hand,their shoulder any little gesture that help them to adjust to physical contact.
Hope this helps.

Insist that he be properly medicated for the aggression or you will resign within 30 days. Stick to that.

I agree that you ought to try to read him and change your technique, but ABOVE ALL keep yourself safe! And, yes, he ought to be evaluated for medication especially if this is a regular occurrence.

Dementia or not, assault is assault.

You used the past tense in your statement. Are you still his caregiver? If so, I'd be very cautious if you're sticking around to care for him. Make sure there are no objects within his reach that he could strike you with. Not that his fists aren't bad enough, but you don't want a lamp over your head or, God forbid, having him pull a gun from his bedside table. Be authoritative in dealing with him.

My child and I endured three months of significant physical and verbal abuse before my husbands care providers believed that the "quiet mild mannered man" with dementia was capable of violence. Finally, he was hospitalized and threw chairs at staff and more. Someone finally believed us and he was medicated. Keep calling the Doctor. Call every time you are abused. Document bruises, cuts, scratches every time they occur. He may not be competent but you are at risk. Take action before you reach the point we did. He is now institutionalize, but the damage is done. Relationships are strained and the child who was is primary target will be in counseling for a long time

Is he able to change his undergarment by himself? A man would have a hard time having a woman do that, and if he is really physically harming you perhaps you need to find another place to live. No caregiver needs to be hit by anyone much less a person with dementia. If you work for an agency, have them send a male or someone else. I was never hit, but plenty of unpleasant moments.