This is quite normal as all his intestines will have been moved and buggered about with to allow the robotic arms unfettered access to the prostate area. Afterwards, they just chuck all the pipework back in place, and then leave nature to take its course so the intestines find their way back to their normal location.
Do persist with the stool-softening drinks such as Fybogel, together with laxatives, and as his GP says, no doubt things will resolve sooner rather than later.
Best wishes.
Cheers, John.