I don't know what to think about what your vet said. I mean he IS the vet, but I've heard of so many cats that have chronic constipation, megacolon, etc...and not cancer along with it.
When I was researching the issue when Paisley had a problem, I thought I saw somewhere that it was one of the most common problems that cats visit the vet for.
Also I saw something about Lactulose. This patient information sheet says it can cause gas, distention of the stomach, cramping, etc.http://www.drsfostersmith.com/Rx_Inf..._lactulose.pdf
Maybe he's experiencing some side effects?
That doesn't explain the red rectum, but I imagine the meds, etc take some time to adjust to. If he did have diarrhea, then that could cause a red rectum too.
They'll often have some soft stool after the enema too, the next day or so...I'm not sure when his diarrhea came on in relation to when he had the enema.
Try to remain positive
...and tell Jordan happy birthday!