I do not like ultimatums, but in this case, I think you have got to give him one!! Get help or Get out.... Especially if there are children involved. Trust me, I KNOW that this can be almost unbelievable, because you love him very much and you know that he is suffering, and you want to be there for him and try to help. But, there is only so much you can do before you INSIST that he take an initiative to get help if he is going to remain a part of your life and your children's lives. These types of disorders are often linked with very agressive and even violent behaviors. We fear for our safety when it comes to our 12 year old daughter's temper tantrums, I cannot IMAGINE dealing with this in a 24 year old man, especially when I am sure that he is much bigger and stronger than you, there is NO way for you to defend yourself against him, and no way to protect your children from him.
Not to mention, they learn by example, especially when they are very young, and could start showing some of this behavior themselves.
Whether or not he has ODD, that is a touchy thing. Generally ODD is a childhood/adolescent issue, because more times than not, if left untreated, ODD turns into Conduct Disorder. I don't have any difficult child's with Conduct Disorder, but with our difficult child's ODD being so serious, we have been cautioned, on several occasions, the severity of this if it turns into Conduct Disorder (CD). Generally CD is so affecting, that the person is unable to function normally, and is usually taking their behavior to such an extreme that the law has to be involved. I have heard of such manifestations of CD that a person with this disorder is likely to be violent and intentionally cruel and torturous to either people or animals, or both. There is a website that contains some information about CD, and also contains very helpful information related to the differences between the two, AND the natural progression from ODD INTO CD as a person grows older.
http://www.hamiltoncenter.org is the website, and from there you click on the Mental Health Information tab, and it will take you to the list and you can select the Conduct Disorder link.