If my partner is aggressive, how do I calm him?

Call 911 and have him transported to the ER for a psych evaluation and given appropriate meds to calm him down. Alzheimer's and dementia can turn ugly and it's important to protect yourself and your children, if they are young and living with both you and your partner.

I agree with Margaret. More information would be helpful.

Are you emotionally or physically being abused? Both are awful.

I sense that this isn’t an occasional ‘off’ day. Anyone can experience being in a bad mood from being in pain or suffering from exhaustion.

Please elaborate what you mean when you say, “aggressive.”

Three days ago you posted a very similar question, which I answered:

"What to do when partner becomes aggressive and is in denial of the whole situation?"

My reply: "To get sensible answers to your question, please could you say what your partner being ‘aggressive’ and ‘hostile’ really means. Nasty words? Sulking? Threats of physical aggression? Actual physical violence? The answers really are different! Please post again. Margaret" 

How about you give us more information?

The safest thing is to remove yourself from where he is until it blows over, go outside, into another room or lock yourself in the bathroom if necessary.

I see from your profile that your partner is fairly young, I'd like to share this video interview with you from another woman dealing with this issue

https://www.youtube.com/watch?v=5LurdJL_voQ