I'm so sorry to hear your kitty isn't feel well.
The symptoms you describe may be due to an anal gland problem. The anal glands are located on either side of the anus, under the skin. They release a foul smelling fluid during each bowel movement. If the fluid becomes thickened, infected, impacted, or is not being released as it should be, this can cause swelling and irritation to the outside of the anus, as well as the anal sacs, inside.
The two white bumps you see, may be due to retention of fluid, and infection. The vet would have to evaluate this area and will empty (express) your cat's anal glands, if necessary, plus, treat the bumps on the outer skin.
It's also possible that your cat, since he goes outside, had sat on an irritating plant, rough surface, etc., and these two bumps are an allergic reaction on the skin only. After a hands-on examination, your vet will make a diagnosis and then prescribe the most appropriate treatment to alleviate the problem.
For the moment, you can apply a hot compress to this area (if your cat lets you), to help draw out infection. Don't stress him out too much, if the area is very tender, and bring him to the vet, tom'w.
I hope your cat is feeling better, very soon!