Sticking to his fur... grooming problem?

So my boy, Hero, has been having problems after using the litter box. He'll come back upstairs and into my room, and I'll notice a smell of "you just went poop" trailing him.

After closer inspection, I notice that he has a lot of poop stuck to him as if he never even bothered to clean himself when finished. The smell is bad because it's right there filling my room, and it becomes a big problem when he sits down and leaves his brown mark on my floor/carpet/bed.

I'm not sure if he just stopped cleaning himself down there or if he's just been having the runs for quite some time now Should I look into getting him trimmed (he's medium hair)?

My girl, River, is one month younger than him, and she is always very clean after she leaves the litter box.

Thanks for the help!

Yes, you could get him a sanitary trim/shave around his butt. It does help a lot. However, if he does have the runs and he's had it more then a couple days, he probably should be checked out by a vet just to rule anything else out.