Hello
*Love_me_Amortentia*. I understand how you feel. I went through a similar situation with my daughter for a few years. Before I give you my story, let me just make sure you understand that I am in no way implying anything about you and your family. I'm just giving my personal situation/experience!
My daughter started off as a sweet, loving, wonderful child. She was happy in every way. After my husband and I got divorced (which is another situation all together

), I had to enlist the help of my mother and step-father when I decided to go back to college. I knew they had their moments and fought with each other. My mother would hit my step-father during some of these fights trying to get more of a rise out of him. However, never in all my wildest dreams did I imagine them doing this in front of my daughter.
For reasons unknown to me at that point, my daughter started to become angry and violent. She started kicking, screaming, hitting, biting, all that stuff. At first, I just thought she was going through a phase brought on by my sudden absences of going to college. At first I tried handling it by talking to her, putting her in time out, little spankings on the butt. It seemed to help a little. But then it started in a roller coaster type ride. Her moods would go up and down in waves of anger and happiness.
This went on for about 2-3 years. I started thinking I was a terrible parent, and that I was to blame and had done something wrong. I thought it may have stemmed back from the reason I had divorced my ex-husband. I thought about sending her to my parent's to live because I didn't think that I knew how to be a mother and raise my child. I was literally at my breaking point.
Finally, after picking up my daughter one evening after class, my daughter told me that Nana hit PeePaw and the police came while she was in the bathroom. I panicked at this point. I called my mother and demanded the truth. Apparently, they had had one of their worse fights that evening. They were yelling at each other with my mother hitting my step-father while my daughter was looking on crying. One of them called the police for some reason. When the police came to the door, my mother put my daughter in the bathroom and told her to stay there and not come out. Luckily, the police left without arresting them.
I completely lost it at that point. I told them that they almost cost me my daughter. If they had been arrested, she would have been put into protective custody because I had allowed her to be exposed to such abuse. They didn't see her again for 6 months. It was almost a year before they were allowed to keep her again. Of course, it was under the strict rule that if I ever find out they had fought in front of her again, they would never see her again.
That first 6 months after that fight were the worst 6 months I ever had with my daughter. She went into complete violence beyond reason. I couldn't control her. I had to break up with my boyfriend of 2-3 years (which we are married now

) to devote all of my attention to my daughter trying to bring her back out of that state. It was horrible.
She's almost 10 now. It took until she was almost 9 to completely get her over all that had happened. Up until then, she still had bouts of anger that we had to deal with. It took a mixture of a lot of love, attention, strict punishments for misbehaving, and rewards for good behavior (and some threats to send her to boot camp) to get her out of that roller coaster ride of emotions.
Looking at her now, you would never guess that she was once that angry and violent. She's completely happy and healthy.
Anyways, not knowing your situation, the only advice I can give is to give your child a lot of love and support and consistant strict punishments when he acts up. Seeing as how he only does this when you go out, is it possible that something may have happened that you don't know about? A babysitter? Daycare? Maybe another kid did something to him? Like I said before, I'm not implying anything. It's just with my daughter, something was happening that I didn't know about and I was blaming myself for it. I'm just trying to make suggestions.