Your son probably has a large blockage of poop but some poop is getting around that and coming out - thus the skid marks even though he's not pooping. You need to get that large blockage taken care of right away. What's happening is that his bowel is growing larger to accommodate the large blockage. Once the bowel enlarges, your son won't feel the need to poop which will cause more constipation. It is a vicious cycle.
My GS had that same problem. What we were told to do, and what worked, was to use a suppository to break up the blockage. After that, we gave him Mineral Oil twice a day as a stool softener. It took a long time to get him back on track because after you get rid of the blockage, the bowel has to shrink again, and that takes time. During that time period, you can't let him get constipated again or the cycle will start all over again, so must do whatever you can to ensure that he poops at least every other day.