How do you control their mood swings?

There could be depression or bi-polar problems here. If that is the case a doctor could prescribe medication. If, after a doctor has decided whether or not medication is indicated, you'll have to try to go with the mood swings, giving the person more "space" when they need it, and encouraging when you can. It's difficult for everyone, but often medication changes can help this.
Carol

I've found that when his mood changes (dark), it's best to change a subject. (What a pretty day!, You look nice in that shirt. A Great day for a walk, etc.) Anything that redirects his thoughts, works sometimes, sometimes not. Sometimes I just go outside a while or in another room, or get him some water to drink or a piece of fruit; just anything that redirects his thoughts.