Bless you for saving this precious feline
life! : )
A swollen rear end and a little bleeding may indicate an irritated rectum/anus, which may be caused by loose stools. Intestinal parasites may cause irritation to the anus and also cause loose or mushy stools.
However, there's another problem which can cause these symptoms--anal sac disease. The anal glands are located on either side of the anus, under the skin. They release a foul-smelling fluid with each bowel movement. If the anal glands become full because the fluid is not being released as usual, they will look swollen, cause the cat discomfort, and there may be some slight bleeding. When the anal glands become full and are not emptying on their own, they must be 'expressed' by the vet, to give the cat relief. Also, the fluid sometimes becomes thickened and/or infected, and the vet will explain what needs to be done if this is the case. http://www.sniksnak.com/cathealth/analsac.html
It's possible that all the medications, including the wormer, is causing some stomach upset and this is irritating his anus when he defecates. If he's 'scooting' (dragging his butt on the floor) at all, this too, is an indication of an anal gland problem or intestinal parasites, which both cause itching and discomfort. It's also possible that he's sensitive to the brand of litter you're using, or a particular brand of detergent or carpet cleaner, if he's sitting on a laundered item like a blanket, etc., a carpeted floor, and his rear is in contact with a substance that's irritating to his skin.
As you can see, there are many possibilities, so if this condition continues, I'd suggest another vet visit, ASAP, so he can be evaluated in person and then treated.
I wish you and your new furry addition much good luck, and please let me know if you need further help in the future. Thanks!