My cat meows and growls badly before having a poo?

Could be the dry food. Boy cats have very very narrow urethers through which the urine pes and dry food, being gritty, has been known to lodge in the urether and cause pain and discomfort when ping urine. I know you say its when he pes solids but something could be pressing on his bladder or whatever, causing discomfort. I would toss the dry food and feed him pouches of wet food, with perhaps an occasional dry treat. You'll soon see a difference if that's the problem.