Yes, it's exactly the same stuff, just a lot cheaper directly from the chemist. I used to use it for Samantha when her bowel muscles became lazy in old age. I often just popped into the chemist for it.
Maybe the pharmacist thought I was anorexic!
However, it does work by drawing water from the body. Can you add water to his food, or persuade him to drink more some other way, to prevent dehydration? I don't know much about CRF, although Samantha's kidneys failed in the last few weeks of her life. Can they eat oil? If they can, I'd suggest either adding a little olive oil to food, or giving tinned fish in oil. I do the latter with mine every month or so to prevent furball. I'm not sure whether this is okay for CRF cats.