Cystitis normally clears up in about 5 days or so (with or without anitbiotics - as it is rare for there to be a bacterial infection that requires antibiotics). As Leanne mentioned, you'll need to keep any eye on him to make sure he is still passing urine, and if he isnt then he will need to go to the vet straight away, but hopefully he will start to feel better in a few days.
Stress in cats is a big contributing factor to urinary problems, it can be something as simple as lack of stimulation, you say the flat is quiet, and he has lots of toys to play with - maybe he's the sort of cat that likes a lot things going on to capture his interest, perhaps he needs more interactive play, as activity is another contributing factor. Diet is another factor, so its good that you've changed him onto wet food to increase his moisture intake, and I would also avoid any wet foods that contain cereals, grains, or vegetables/vegetable proteins as these can excacerbat the problem.
