Sounds to me like something definitely is wrong in his life. Either with mom or at your guys' house or at school? I think you are right to be concerned. Also right that dad needs to take this very seriously.
The one thing that struck me, that no one else has pointed out yet-- I am hesitant to write this in fear of hurting your feelings BUT here it goes--is that you said your DH's attitude is that you are "picking on him because he is not one of my biological kids" Why does dad feel this way? Are you treating him differently than your other kids, especially since he is the only "step" kid left in your house? Could your stepson feel as if you are "picking on him"? If dad feels this way maybe their is some tiny truth to it?
Hope you take a good hard look at EVERYTHING going on in his life, not just at his mom's like some of the others have said...I say this because he is not talking about burning things at his mom's house, all his anger seems to be directed at your house and your kid(s)??
I agree that the list shows intent and planning...which is never good in a situation like this. A family pow-wow/sit down is probably in order.....or even just a special moment between you and stepson, where you can express you love and fears about his unhappiness in your house.
Good Luck. Being the step parent is very hard. Don't give up and keep on top of things and an extra eye on him....especially after reading some of the other posts about "cutting" or self mutilation.
I am not saying any of this to "bash" you, I am just giving you another point of view, feel free to disregard it all!