My first though is a splinter./ sticker. I know they swell up.
I haven't been around except for short sections of time, so I haven't witnessed the natural process, but since this bump is a couple weeks old I assume he would be displaying some sign of discomfort if he was blocked.It’s hard for me to tell in the pic, but is it right beside his anus? Perhaps a scent gland got blocked up?
Is he pooping ok- nothings being blocked?