I would get him to the vets straight awy for a check up. if your anus was bleeding and sore you would seek treatment
Actually we see impacted anal glands in older kittens at the shelter a lot. If they are still swollen I wouldn't wait to take him to the vet. Also, the diarrhea can cause dehydration very quickly, particularly in a kitten, even if they are still drinking an eating. Since he goes out it could also be from a wide variety of worms or other parasites. Most are not visible to the naked eye. In the absence of parasites causing the diarrhea, I have had great success is using probiotics (available as liquid or powder) for feline diarrhea.
he is licking his anus b/c it feels uncomfortable from the irritation. has he had a change in his diet recently? or perhaps since he does go outside 4 hrs a day, could he have eaten something that didn't agree with him? Has he had vaccinations recently? all that could be the cause of the diarrhea.
His anal glands could also be plugged although this isn't usual in a cat so young....but if the anal glands continue to be swollen it would be a good idea to have a Vet check hm out.
IF the diarrhea clears within a day or two and the anal glands return to normal than I would assume its something minor, but I wouldn't let the diarrhea persist longer than that, unless you can trace the cause to a change in diet or something I mentioned.
please keep us updated, perhaps someone else on the forum also has some suggestions. good luck♥