I am so glad you are concerned for your stepson, rather than angry at him. It is a fairly common problem. The fecal soiling is called encopresis, and the urination is called enuresis. Most pediatricians don't seem to have a real thorough understanding of the problem or a solution.
My daughter has struggled with encopresis, and it has taken me a long time to grasp it (I would get mad at her, bless her heart). When I realized it was beyond her control, I found a great source of help, via the internet. Go to www.soilingsolutions.com and read the website. I ordered the book for encopresis and we've used the program - it helps tremendously! There are still setbacks...I think personality plays a role, in the sense that my 6 year old would rather play and play and not take time to go to the bathroom. She then becomes constipated, then leaks around it but doesn't feel the leakage. So it is a matter of retraining her bowel muscles as well as retraining her behavior. It takes consistency and patience, and I have to admit I don't always have those qualities myself! (so I shouldn't blame her!) At any rate, I recommend you approach this matter-of-factly. Make him sit on the potty every hour or so, so he can begin to get in touch with the sensation of needing to go and the self-discipline to go before urgently needing to! And matter-of-factly have him clean himself up and wash out his soiled underwear, then run them through the washer. It will take lots of time - just as if you're potty training a toddler, because you are, kind of. But also consider looking at the website for soiling solutions, because the program works very well. It doesn't get into all the possible psychological issues, which I think is good. It just addresses the behavior and retrains the child. God bless you both!